This page is a tutorial of the Structured Query Language (also known as SQL) and is a pioneering effort on the World Wide Web, as this is the first comprehensive SQL tutorial available on the Internet. SQL allows users to access data in relational database management systems, such as Oracle, Sybase, Informix, Microsoft SQL Server, Access, and others, by allowing users to describe the data the user wishes to see. SQL also allows users to define the data in a database, and manipulate that data. This page will describe how to use SQL, and give examples. The SQL used in this document is "ANSI", or standard SQL, and no SQL features of specific database management systems will be discussed until the "Nonstandard SQL" section. It is recommended that you print this page, so that you can easily refer back to previous examples.
Also, you may be interested in joining the new SQL Club on Yahoo!, where you can read or enter messages in a SQL forum.
Performing a Join
Aliases & In/Subqueries
Creating New Tables
GROUP BY & HAVING
EXISTS & ALL
UNION & Outer Joins
Common SQL Questions
In a relational database, data is stored in tables. An example table would relate Social Security Number, Name, and Address:
|512687458||Joe||Smith||83 First Street||Howard||Ohio|
|758420012||Mary||Scott||842 Vine Ave.||Losantiville||Ohio|
|102254896||Sam||Jones||33 Elm St.||Paris||New York|
|876512563||Sarah||Ackerman||440 U.S. 110||Upton||Michigan|
Now, let's say you want to see the address of each employee. Use the SELECT statement, like so:
SELECT FirstName, LastName, Address, City, State
The following is the results of your query of the database:
|First Name||Last Name||Address||City||State|
|Joe||Smith||83 First Street||Howard||Ohio|
|Mary||Scott||842 Vine Ave.||Losantiville||Ohio|
|Sam||Jones||33 Elm St.||Paris||New York|
|Sarah||Ackerman||440 U.S. 110||Upton||Michigan|
To explain what you just did, you asked for the all of data in the EmployeeAddressTable, and specifically, you asked for the columns called FirstName, LastName, Address, City, and State. Note that column names and table names do not have spaces...they must be typed as one word; and that the statement ends with a semicolon (;). The general form for a SELECT statement, retrieving all of the rows in the table is:
SELECT ColumnName, ColumnName, ...
To get all columns of a table without typing all column names, use:
SELECT * FROM TableName;
Each database management system (DBMS) and database software has different methods for logging in to the database and entering SQL commands; see the local computer "guru" to help you get onto the system, so that you can use SQL.
To further discuss the SELECT statement, let's look at a new example table (for hypothetical purposes only):
|< or != (see manual)||Not Equal|
|<=||Less Than or Equal To|
|>=||Greater Than or Equal To|
The WHERE clause is used to specify that only certain rows of the table are displayed, based on the criteria described in that WHERE clause. It is most easily understood by looking at a couple of examples.
If you wanted to see the EMPLOYEEIDNO's of those making at or over $50,000, use the following:
WHERE SALARY >= 50000;
Notice that the >= (greater than or equal to) sign is used, as we wanted to see those who made greater than $50,000, or equal to $50,000, listed together. This displays:
The WHERE description, SALARY >= 50000, is known as a condition (an operation which evaluates to True or False). The same can be done for text columns:
WHERE POSITION = 'Manager';
This displays the ID Numbers of all Managers. Generally, with text columns, stick to equal to or not equal to, and make sure that any text that appears in the statement is surrounded by single quotes ('). Note: Position is now an illegal identifier because it is now an unused, but reserved, keyword in the SQL-92 standard.
The AND operator joins two or more conditions, and displays a row only if that row's data satisfies ALL conditions listed (i.e. all conditions hold true). For example, to display all staff making over $40,000, use:
WHERE SALARY > 40000 AND POSITION = 'Staff';
The OR operator joins two or more conditions, but returns a row if ANY of the conditions listed hold true. To see all those who make less than $40,000 or have less than $10,000 in benefits, listed together, use the following query:
WHERE SALARY < 40000 OR BENEFITS < 10000;
AND & OR can be combined, for example:
WHERE POSITION = 'Manager' AND SALARY > 60000 OR BENEFITS > 12000;
First, SQL finds the rows where the salary is greater than $60,000 and the position column is equal to Manager, then taking this new list of rows, SQL then sees if any of these rows satisfies the previous AND condition or the condition that the Benefits column is greater than $12,000. Subsequently, SQL only displays this second new list of rows, keeping in mind that anyone with Benefits over $12,000 will be included as the OR operator includes a row if either resulting condition is True. Also note that the AND operation is done first.
To generalize this process, SQL performs the AND operation(s) to determine the rows where the AND operation(s) hold true (remember: all of the conditions are true), then these results are used to compare with the OR conditions, and only display those remaining rows where any of the conditions joined by the OR operator hold true (where a condition or result from an AND is paired with another condition or AND result to use to evaluate the OR, which evaluates to true if either value is true). Mathematically, SQL evaluates all of the conditions, then evaluates the AND "pairs", and then evaluates the OR's (where both operators evaluate left to right).
To look at an example, for a given row for which the DBMS is evaluating the SQL statement Where clause to determine whether to include the row in the query result (the whole Where clause evaluates to True), the DBMS has evaluated all of the conditions, and is ready to do the logical comparisons on this result:
True AND False OR True AND True OR False AND False
First simplify the AND pairs:
False OR True OR False
Now do the OR's, left to right:
True OR False
The result is True, and the row passes the query conditions. Be sure to see the next section on NOT's, and the order of logical operations. I hope that this section has helped you understand AND's or OR's, as it's a difficult subject to explain briefly.
To perform OR's before AND's, like if you wanted to see a list of employees making a large salary ($50,000) or have a large benefit package ($10,000), and that happen to be a manager, use parentheses:
WHERE POSITION = 'Manager' AND (SALARY > 50000 OR BENEFITS > 10000);
IN & BETWEEN
An easier method of using compound conditions uses IN or BETWEEN. For example, if you wanted to list all managers and staff:
WHERE POSITION IN ('Manager', 'Staff');
or to list those making greater than or equal to $30,000, but less than or equal to $50,000, use:
WHERE SALARY BETWEEN 30000 AND 50000;
To list everyone not in this range, try:
WHERE SALARY NOT BETWEEN 30000 AND 50000;
Similarly, NOT IN lists all rows excluded from the IN list.
Additionally, NOT's can be thrown in with AND's & OR's, except that NOT is a unary operator (evaluates one condition, reversing its value, whereas, AND's & OR's evaluate two conditions), and that all NOT's are performed before any AND's or OR's.
SQL Order of Logical Operations (each operates from left to right)
Look at the EmployeeStatisticsTable, and say you wanted to see all people whose last names started with "S"; try:
WHERE LASTNAME LIKE 'S%';
The percent sign (%) is used to represent any possible character (number, letter, or punctuation) or set of characters that might appear after the "S". To find those people with LastName's ending in "S", use '%S', or if you wanted the "S" in the middle of the word, try '%S%'. The '%' can be used for any characters in the same position relative to the given characters. NOT LIKE displays rows not fitting the given description. Other possibilities of using LIKE, or any of these discussed conditionals, are available, though it depends on what DBMS you are using; as usual, consult a manual or your system manager or administrator for the available features on your system, or just to make sure that what you are trying to do is available and allowed. This disclaimer holds for the features of SQL that will be discussed below. This section is just to give you an idea of the possibilities of queries that can be written in SQL.
In this section, we will only discuss inner joins, and equijoins, as in general, they are the most useful. For more information, try the SQL links at the bottom of the page.
Good database design suggests that each table lists data only about a single entity, and detailed information can be obtained in a relational database, by using additional tables, and by using a join.
First, take a look at these example tables:
First, let's discuss the concept of keys. A primary key is a column or set of columns that uniquely identifies the rest of the data in any given row. For example, in the AntiqueOwners table, the OwnerID column uniquely identifies that row. This means two things: no two rows can have the same OwnerID, and, even if two owners have the same first and last names, the OwnerID column ensures that the two owners will not be confused with each other, because the unique OwnerID column will be used throughout the database to track the owners, rather than the names.
A foreign key is a column in a table where that column is a primary key of another table, which means that any data in a foreign key column must have corresponding data in the other table where that column is the primary key. In DBMS-speak, this correspondence is known as referential integrity. For example, in the Antiques table, both the BuyerID and SellerID are foreign keys to the primary key of the AntiqueOwners table (OwnerID; for purposes of argument, one has to be an Antique Owner before one can buy or sell any items), as, in both tables, the ID rows are used to identify the owners or buyers and sellers, and that the OwnerID is the primary key of the AntiqueOwners table. In other words, all of this "ID" data is used to refer to the owners, buyers, or sellers of antiques, themselves, without having to use the actual names.
The purpose of these keys is so that data can be related across tables, without having to repeat data in every table--this is the power of relational databases. For example, you can find the names of those who bought a chair without having to list the full name of the buyer in the Antiques table...you can get the name by relating those who bought a chair with the names in the AntiqueOwners table through the use of the OwnerID, which relates the data in the two tables. To find the names of those who bought a chair, use the following query:
SELECT OWNERLASTNAME, OWNERFIRSTNAME
FROM ANTIQUEOWNERS, ANTIQUES
WHERE BUYERID = OWNERID AND ITEM = 'Chair';
Note the following about this query...notice that both tables involved in the relation are listed in the FROM clause of the statement. In the WHERE clause, first notice that the ITEM = 'Chair' part restricts the listing to those who have bought (and in this example, thereby owns) a chair. Secondly, notice how the ID columns are related from one table to the next by use of the BUYERID = OWNERID clause. Only where ID's match across tables and the item purchased is a chair (because of the AND), will the names from the AntiqueOwners table be listed. Because the joining condition used an equal sign, this join is called an equijoin. The result of this query is two names: Smith, Bob & Fowler, Sam.
Dot notation refers to prefixing the table names to column names, to avoid ambiguity, as follows:
SELECT ANTIQUEOWNERS.OWNERLASTNAME, ANTIQUEOWNERS.OWNERFIRSTNAME
FROM ANTIQUEOWNERS, ANTIQUES
WHERE ANTIQUES.BUYERID = ANTIQUEOWNERS.OWNERID AND ANTIQUES.ITEM = 'Chair';
As the column names are different in each table, however, this wasn't necessary.
Let's say that you want to list the ID and names of only those people who have sold an antique. Obviously, you want a list where each seller is only listed once--you don't want to know how many antiques a person sold, just the fact that this person sold one (for counts, see the Aggregate Function section below). This means that you will need to tell SQL to eliminate duplicate sales rows, and just list each person only once. To do this, use the DISTINCT keyword.
First, we will need an equijoin to the AntiqueOwners table to get the detail data of the person's LastName and FirstName. However, keep in mind that since the SellerID column in the Antiques table is a foreign key to the AntiqueOwners table, a seller will only be listed if there is a row in the AntiqueOwners table listing the ID and names. We also want to eliminate multiple occurrences of the SellerID in our listing, so we use DISTINCT on the column where the repeats may occur (however, it is generally not necessary to strictly put the Distinct in front of the column name).
To throw in one more twist, we will also want the list alphabetized by LastName, then by FirstName (on a LastName tie). Thus, we will use the ORDER BY clause:
SELECT DISTINCT SELLERID, OWNERLASTNAME, OWNERFIRSTNAME
FROM ANTIQUES, ANTIQUEOWNERS
WHERE SELLERID = OWNERID
ORDER BY OWNERLASTNAME, OWNERFIRSTNAME;
In this example, since everyone has sold an item, we will get a listing of
all of the owners, in alphabetical order by last name. For future reference (and
in case anyone asks), this type of join is considered to be in the category of
Aliases & In/Subqueries
In this section, we will talk about Aliases, In and the use of subqueries, and how these can be used in a 3-table example. First, look at this query which prints the last name of those owners who have placed an order and what the order is, only listing those orders which can be filled (that is, there is a buyer who owns that ordered item):
SELECT OWN.OWNERLASTNAME Last Name, ORD.ITEMDESIRED Item Ordered
FROM ORDERS ORD, ANTIQUEOWNERS OWN
WHERE ORD.OWNERID = OWN.OWNERID
AND ORD.ITEMDESIRED IN
Last Name Item Ordered
There are several things to note about this query:
I will discuss five important aggregate functions: SUM, AVG, MAX, MIN, and COUNT. They are called aggregate functions because they summarize the results of a query, rather than listing all of the rows.
SELECT SUM(SALARY), AVG(SALARY)
This query shows the total of all salaries in the table, and the average salary of all of the entries in the table.
WHERE POSITION = 'Manager';
This query gives the smallest figure of the Benefits column, of the employees who are Managers, which is 12500.
WHERE POSITION = 'Staff';
This query tells you how many employees have Staff status (3).
In SQL, you might (check your DBA) have access to create views for yourself. What a view does is to allow you to assign the results of a query to a new, personal table, that you can use in other queries, where this new table is given the view name in your FROM clause. When you access a view, the query that is defined in your view creation statement is performed (generally), and the results of that query look just like another table in the query that you wrote invoking the view. For example, to create a view:
CREATE VIEW ANTVIEW AS SELECT ITEMDESIRED FROM ORDERS;
Now, write a query using this view as a table, where the table is just a listing of all Items Desired from the Orders table:
FROM ANTIQUES, ANTVIEW
WHERE ITEMDESIRED = ITEM;
This query shows all SellerID's from the Antiques table where the Item in that table happens to appear in the Antview view, which is just all of the Items Desired in the Orders table. The listing is generated by going through the Antique Items one-by-one until there's a match with the Antview view. Views can be used to restrict database access, as well as, in this case, simplify a complex query.
All tables within a database must be created at some point in time...let's see how we would create the Orders table:
CREATE TABLE ORDERS
(OWNERID INTEGER NOT NULL,
ITEMDESIRED CHAR(40) NOT NULL);
This statement gives the table name and tells the DBMS about each column in the table. Please note that this statement uses generic data types, and that the data types might be different, depending on what DBMS you are using. As usual, check local listings. Some common generic data types are:
Let's add a column to the Antiques table to allow the entry of the price of a given Item:
ALTER TABLE ANTIQUES ADD (PRICE DECIMAL(8,2) NULL);
The data for this new column can be updated or inserted as shown later.
To insert rows into a table, do the following:
INSERT INTO ANTIQUES VALUES (21, 01, 'Ottoman', 200.00);
This inserts the data into the table, as a new row, column-by-column, in the pre-defined order. Instead, let's change the order and leave Price blank:
INSERT INTO ANTIQUES (BUYERID, SELLERID, ITEM)
VALUES (01, 21, 'Ottoman');
Let's delete this new row back out of the database:
DELETE FROM ANTIQUES
WHERE ITEM = 'Ottoman';
But if there is another row that contains 'Ottoman', that row will be deleted also. Let's delete all rows (one, in this case) that contain the specific data we added before:
DELETE FROM ANTIQUES
WHERE ITEM = 'Ottoman' AND BUYERID = 01 AND SELLERID = 21;
Let's update a Price into a row that doesn't have a price listed yet:
UPDATE ANTIQUES SET PRICE = 500.00 WHERE ITEM = 'Chair';
This sets all Chair's Prices to 500.00. As shown above, more WHERE conditionals, using AND, must be used to limit the updating to more specific rows. Also, additional columns may be set by separating equal statements with commas.
Indexes allow a DBMS to access data quicker (please note: this feature is nonstandard/not available on all systems). The system creates this internal data structure (the index) which causes selection of rows, when the selection is based on indexed columns, to occur faster. This index tells the DBMS where a certain row is in the table given an indexed-column value, much like a book index tells you what page a given word appears. Let's create an index for the OwnerID in the AntiqueOwners column:
CREATE INDEX OID_IDX ON ANTIQUEOWNERS (OWNERID);
Now on the names:
CREATE INDEX NAME_IDX ON ANTIQUEOWNERS (OWNERLASTNAME, OWNERFIRSTNAME);
To get rid of an index, drop it:
DROP INDEX OID_IDX;
By the way, you can also "drop" a table, as well (careful!--that means that your table is deleted). In the second example, the index is kept on the two columns, aggregated together--strange behavior might occur in this situation...check the manual before performing such an operation.
Some DBMS's do not enforce primary keys; in other words, the uniqueness of a column is not enforced automatically. What that means is, if, for example, I tried to insert another row into the AntiqueOwners table with an OwnerID of 02, some systems will allow me to do that, even though we do not, as that column is supposed to be unique to that table (every row value is supposed to be different). One way to get around that is to create a unique index on the column that we want to be a primary key, to force the system to enforce prohibition of duplicates:
CREATE UNIQUE INDEX OID_IDX ON ANTIQUEOWNERS (OWNERID);
GROUP BY & HAVING
One special use of GROUP BY is to associate an aggregate function (especially COUNT; counting the number of rows in each group) with groups of rows. First, assume that the Antiques table has the Price column, and each row has a value for that column. We want to see the price of the most expensive item bought by each owner. We have to tell SQL to group each owner's purchases, and tell us the maximum purchase price:
SELECT BUYERID, MAX(PRICE)
GROUP BY BUYERID;
Now, say we only want to see the maximum purchase price if the purchase is over $1000, so we use the HAVING clause:
SELECT BUYERID, MAX(PRICE)
GROUP BY BUYERID
HAVING PRICE 1000;
Another common usage of subqueries involves the use of operators to allow a Where condition to include the Select output of a subquery. First, list the buyers who purchased an expensive item (the Price of the item is $100 greater than the average price of all items purchased):
WHERE PRICE >
List the Last Names of those in the AntiqueOwners table, ONLY if they have bought an item:
WHERE OWNERID IN
For an Update example, we know that the gentleman who bought the bookcase has the wrong First Name in the database...it should be John:
SET OWNERFIRSTNAME = 'John'
WHERE OWNERID =
Remember this rule about subqueries: when you have a subquery as part of a WHERE condition, the Select clause in the subquery must have columns that match in number and type to those in the Where clause of the outer query. In other words, if you have "WHERE ColumnName = (SELECT...);", the Select must have only one column in it, to match the ColumnName in the outer Where clause, and they must match in type (both being integers, both being character strings, etc.).
EXISTS uses a subquery as a condition, where the condition is True if the subquery returns any rows, and False if the subquery does not return any rows; this is a nonintuitive feature with few unique uses. However, if a prospective customer wanted to see the list of Owners only if the shop dealt in Chairs, try:
SELECT OWNERFIRSTNAME, OWNERLASTNAME
ALL is another unusual feature, as ALL queries can usually be done with different, and possibly simpler methods; let's take a look at an example query:
SELECT BUYERID, ITEM
WHERE PRICE = ALL
There are occasions where you might want to see the results of multiple queries together, combining their output; use UNION. To merge the output of the following two queries, displaying the ID's of all Buyers, plus all those who have an Order placed:
Notice that SQL requires that the Select list (of columns) must match, column-by-column, in data type. In this case BuyerID and OwnerID are of the same data type (integer). Also notice that SQL does automatic duplicate elimination when using UNION (as if they were two "sets"); in single queries, you have to use DISTINCT.
The outer join is used when a join query is "united" with the rows not included in the join, and are especially useful if constant text "flags" are included. First, look at the query:
SELECT OWNERID, 'is in both Orders & Antiques'
FROM ORDERS, ANTIQUES
WHERE OWNERID = BUYERID
SELECT BUYERID, 'is in Antiques only'
WHERE BUYERID NOT IN
This concept is useful in situations where a primary key is related to a foreign key, but the foreign key value for some primary keys is NULL. For example, in one table, the primary key is a salesperson, and in another table is customers, with their salesperson listed in the same row. However, if a salesperson has no customers, that person's name won't appear in the customer table. The outer join is used if the listing of all salespersons is to be printed, listed with their customers, whether the salesperson has a customer or not--that is, no customer is printed (a logical NULL value) if the salesperson has no customers, but is in the salespersons table. Otherwise, the salesperson will be listed with each customer.
Another important related point about Nulls having to do with joins: the order of tables listed in the From clause is very important. The rule states that SQL "adds" the second table to the first; the first table listed has any rows where there is a null on the join column displayed; if the second table has a row with a null on the join column, that row from the table listed second does not get joined, and thus included with the first table's row data. This is another occasion (should you wish that data included in the result) where an outer join is commonly used. The concept of nulls is important, and it may be worth your time to investigate them further.
ENOUGH QUERIES!!! you say?...now on to something completely different...
/* -To get right to it, here is an example program that uses
SQL. Embedded SQL allows programmers to connect to a database and
include SQL code right in the program, so that their programs can
use, manipulate, and process data from a database.
-This example C Program (using Embedded SQL) will print a report.
-This program will have to be precompiled for the SQL statements,
before regular compilation.
-The EXEC SQL parts are the same (standard), but the surrounding C
code will need to be changed, including the host variable
declarations, if you are using a different language.
-Embedded SQL changes from system to system, so, once again, check
local documentation, especially variable declarations and logging
in procedures, in which network, DBMS, and operating system
considerations are crucial. */
/* THIS PROGRAM IS NOT COMPILABLE OR EXECUTABLE */
/* IT IS FOR EXAMPLE PURPOSES ONLY */
/* This section declares the host variables; these will be the
variables your program uses, but also the variable SQL will put
values in or take values out. */
EXEC SQL BEGIN DECLARE SECTION;
char FirstName, LastName, Item;
EXEC SQL END DECLARE SECTION;
/* This includes the SQLCA variable, so that some error checking can be
EXEC SQL INCLUDE SQLCA;
/* This is a possible way to log into the database */
EXEC SQL CONNECT UserID/Password;
/* This code either says that you are connected or checks if an
code was generated, meaning log in was incorrect or not possible. */
printf(Printer, "Error connecting to database server.\n");
printf("Connected to database server.\n");
/* This declares a "Cursor". This is used when a query returns more
than one row, and an operation is to be performed on each row
resulting from the query. With each row established by this query,
I'm going to use it in the report. Later, "Fetch" will be used to
pick off each row, one at a time, but for the query to actually
be executed, the "Open" statement is used. The "Declare" just
establishes the query. */
EXEC SQL DECLARE ItemCursor CURSOR FOR
SELECT ITEM, BUYERID
ORDER BY ITEM;
EXEC SQL OPEN ItemCursor;
/* +-- You may wish to put a similar error checking block here --+ */
/* Fetch puts the values of the "next" row of the query in the host
variables, respectively. However, a "priming fetch" (programming
technique) must first be done. When the cursor is out of data, a
sqlcode will be generated allowing us to leave the loop. Notice
that, for simplicity's sake, the loop will leave on any sqlcode,
even if it is an error code. Otherwise, specific code checking must
be performed. */
EXEC SQL FETCH ItemCursor INTO :Item, :BuyerID;
/* With each row, we will also do a couple of things. First, bump
price up by $5 (dealer's fee) and get the buyer's name to put in
the report. To do this, I'll use an Update and a Select, before
printing the line on the screen. The update assumes however, that
a given buyer has only bought one of any given item, or else the
price will be increased too many times. Otherwise, a "RowID" logic
would have to be used (see documentation). Also notice the colon
before host variable names when used inside of SQL statements. */
EXEC SQL UPDATE ANTIQUES
SET PRICE = PRICE + 5
WHERE ITEM = :Item AND BUYERID = :BuyerID;
EXEC SQL SELECT OWNERFIRSTNAME, OWNERLASTNAME
INTO :FirstName, :LastName
WHERE BUYERID = :BuyerID;
printf("%25s %25s %25s", FirstName, LastName, Item);
/* Ugly report--for example purposes only! Get the next row. */
EXEC SQL FETCH ItemCursor INTO :Item, :BuyerID;
/* Close the cursor, commit the changes (see below), and exit the
EXEC SQL CLOSE ItemCursor;
EXEC SQL COMMIT RELEASE;
Common SQL Questions & Advanced Topics
Also, if you wish to practice SQL on an interactive site (using Java technologies), I highly recommend Frank Torres' (firstname.lastname@example.org) site at http://sqlcourse.com and its new sequel (so to speak) site at http://sqlcourse2.com. Frank did an outstanding job with his site, and if you have a recent release browser, it's definitely worth a visit. In addition, point your browser to http://www.topica.com/, and subscribe to their SQL e-mail Tips of the Day...they are outstanding; Tim Quinlan goes into topics that I can't even begin to go into here, such index data structures (B-trees and B+-trees) and join algorithms, so advanced IT RDBMS pros will get a daily insight into these data management tools.
Unfortunately, there is not a great deal of information on the web about SQL; the list I have below is fairly comprehensive (definitely representative). As far as books are concerned, I would suggest (for beginners to intermediate-level) "Oracle: The Complete Reference" (multiple versions) from Oracle and "Understanding SQL" from Sybex for general SQL information. Also, I would recommend O'Reilly Publishing's books, and Joe Celko's writings for advanced users. For specific DBMS info (especially in the Access area), I recommend Que's "Using" series, and the books of Alison Balter.
Select ...DECODE (Value, If1, Then1, [If 2, Then 2, ...,] Else) ...From ...;
The Value is the name of a column, or a function (conceivably based on a column or columns), and for each If included in the statement, the corresponding Then clause is the output if the condition is true. If none of the conditions are true, then the Else value is output. Let's look at an example:
Select Distinct City,
DECODE (City, 'Cincinnati', 'Queen City', 'New York', 'Big Apple', 'Chicago',
'City of Broad Shoulders', City) AS Nickname
The output might look like this:
Cincinnati Queen City
New York Big Apple
'City' in the first argument denotes the column name used for the test. The second, fourth, etc. arguments are the individual equality tests (taken in the order given) against each value in the City column. The third, fifth, etc. arguments are the corresponding outputs if the corresponding test is true. The final parameter is the default output if none of the tests are true; in this case, just print out the column value.
TIP: If you want nothing to be output for a given condition, such as the default "Else" value, enter the value Null for that value, such as:
Select Distinct City,
DECODE (City, 'Cincinnati', 'Queen City', 'New York', 'Big Apple', 'Chicago',
'City of Broad Shoulders', Null) AS Nickname
If the City column value is not one of the ones mentioned, nothing is outputted, rather than the city name itself.
Cincinnati Queen City
New York Big Apple
For example, Microsoft SQL Server (7.0 & below) requires that you write "triggers" (see the Yahoo SQL Club link to find links that discuss this topic--I may include that topic in a future version of this page) to implement this. (A quick definition, though; a Trigger is a SQL statement stored in the database that allows you to perform a given query [usually an "Action" Query--Delete, Insert, Update] automatically, when a specified event occurs in the database, such as a column update, but anyway...) Microsoft Access (believe it or not) will perform this if you define it in the Relationships screen, but it will still burden you with a prompt. Oracle does this automatically, if you specify a special "Constraint" (see reference at bottom for definition, not syntax) on the keyed column.
So, I'll just discuss the concept. First, see the discussion above on Primary and Foreign keys.
Concept: If a row from the primary key column is deleted/updated, if "Cascading" is activated, the value of the foreign key in those other tables will be deleted (the whole row)/updated.
The reverse, a foreign key deletion/update causing a primary key value to be deleted/changed, may or may not occur: the constraint or trigger may not be defined, a "one-to-many" relationship may exist, the update might be to another existing primary key value, or the DBMS itself may or may not have rules governing this. As usual, see your DBMS's documentation.
For example, if you set up the AntiqueOwners table to have a Primary Key, OwnerID, and you set up the database to delete rows on the Foreign Key, SellerID, in the Antiques table, on a primary key deletion, then if you deleted the AntiqueOwners row with OwnerID of '01', then the rows in Antiques, with the Item values, Bed, Cabinet, and Jewelry Box ('01' sold them), will all be deleted. Of course, assuming the proper DB definition, if you just updated '01' to another value, those Seller ID values would be updated to that new value too.
Think of the following Employee table (the employees are given numbers, for simplicity):
Now think of a department table:
Now suppose you want to join the tables, seeing all of the employees and all of the departments together...you'll have to use an outer join which includes a null employee to go with Dept. 40.
In the book, "Oracle 7: the Complete Reference", about outer joins, "think of the (+), which must immediately follow the join column of the table, as saying add an extra (null) row anytime there's no match". So, in Oracle, try this query (the + goes on Employee, which adds the null row on no match):
Select E.Name, D.Department
From Department D, Employee E
Where E.Department(+) = D.Department;
This is a left (outer) join, in Access:
SELECT DISTINCTROW Employee.Name, Department.Department
FROM Department LEFT JOIN Employee ON Department.Department = Employee.Department;
And you get this result:
FROM AntiqueOwners, Orders;
If you think about it, you can see how joins work. Look at the Cartesian product results, then look for rows where the OwnerID's are equal, and the result is what you would get on an equijoin.
Of course, this is not how DBMS's actually perform joins because loading this result can take too much memory; instead, comparisons are performed in nested loops, or by comparing values in indexes, and then loading result rows.
First Normal Form refers to moving data into separate tables where the data in each table is of a similar type, and by giving each table a primary key.
Putting data in Second Normal Form involves removing to other tables data that is only dependent of a part of the key. For example, if I had left the names of the Antique Owners in the items table, that would not be in Second Normal Form because that data would be redundant; the name would be repeated for each item owned; as such, the names were placed in their own table. The names themselves don't have anything to do with the items, only the identities of the buyers and sellers.
Third Normal Form involves getting rid of anything in the tables that doesn't depend solely on the primary key. Only include information that is dependent on the key, and move off data to other tables that are independent of the primary key, and create a primary key for the new tables.
There is some redundancy to each form, and if data is in 3NF (shorthand for 3rd normal form), it is already in 1NF and 2NF. In terms of data design then, arrange data so that any non-primary key columns are dependent only on the whole primary key. If you take a look at the sample database, you will see that the way then to navigate through the database is through joins using common key columns.
Two other important points in database design are using good, consistent, logical, full-word names for the tables and columns, and the use of full words in the database itself. On the last point, my database is lacking, as I use numeric codes for identification. It is usually best, if possible, to come up with keys that are, by themselves, self-explanatory; for example, a better key would be the first four letters of the last name and first initial of the owner, like JONEB for Bill Jones (or for tiebreaking purposes, add numbers to the end to differentiate two or more people with similar names, so you could try JONEB1, JONEB2, etc.).
Three reasons immediately come to mind as to why this is important. First, getting multiple rows when you were expecting only one, or vice-versa, may mean that the query is erroneous, that the database is incomplete, or simply, you learned something new about your data. Second, if you are using an update or delete statement, you had better be sure that the statement that you write performs the operation on the desired row (or rows)...or else, you might be deleting or updating more rows than you intend. Third, any queries written in Embedded SQL must be carefully thought out as to the number of rows returned. If you write a single-row query, only one SQL statement may need to be performed to complete the programming logic required. If your query, on the other hand, returns multiple rows, you will have to use the Fetch statement, and quite probably, some sort of looping structure in your program will be required to iterate processing on each returned row of the query.
First, create a list of important things (entities) and include those things you may not initially believe is important. Second, draw a line between any two entities that have any connection whatsoever; except that no two entities can connect without a 'rule'; e.g.: families have children, employees work for a department. Therefore put the 'connection' in a diamond, the 'entities' in squares. Third, your picture should now have many squares (entities) connected to other entities through diamonds (a square enclosing an entity, with a line to a diamond describing the relationship, and then another line to the other entity). Fourth, put descriptors on each square and each diamond, such as customer -- airline -- trip. Fifth, give each diamond and square any attributes it may have (a person has a name, an invoice has a number), but some relationships have none (a parent just owns a child). Sixth, everything on your page that has attributes is now a table, whenever two entities have a relationship where the relationship has no attributes, there is merely a foreign key between the tables. Seventh, in general you want to make tables not repeat data. So, if a customer has a name and several addresses, you can see that for every address of a customer, there will be repeated the customer's first name, last name, etc. So, record Name in one table, and put all his addresses in another. Eighth, each row (record) should be unique from every other one; Mr. Freedman suggests a 'auto-increment number' primary key, where a new, unique number is generated for each new inserted row. Ninth, a key is any way to uniquely identify a row in a table...first and last name together are good as a 'composite' key. That's the technique.
A One-to-one relationship means that you have a primary key column that is related to a foreign key column, and that for every primary key value, there is one foreign key value. For example, in the first example, the EmployeeAddressTable, we add an EmployeeIDNo column. Then, the EmployeeAddressTable is related to the EmployeeStatisticsTable (second example table) by means of that EmployeeIDNo. Specifically, each employee in the EmployeeAddressTable has statistics (one row of data) in the EmployeeStatisticsTable. Even though this is a contrived example, this is a "1-1" relationship. Also notice the "has" in bold...when expressing a relationship, it is important to describe the relationship with a verb.
The other two kinds of relationships may or may not use logical primary key and foreign key constraints...it is strictly a call of the designer. The first of these is the one-to-many relationship ("1-M"). This means that for every column value in one table, there is one or more related values in another table. Key constraints may be added to the design, or possibly just the use of some sort of identifier column may be used to establish the relationship. An example would be that for every OwnerID in the AntiqueOwners table, there are one or more (zero is permissible too) Items bought in the Antiques table (verb: buy).
Finally, the many-to-many relationship ("M-M") does not involve keys generally, and usually involves identifying columns. The unusual occurrence of a "M-M" means that one column in one table is related to another column in another table, and for every value of one of these two columns, there are one or more related values in the corresponding column in the other table (and vice-versa), or more a common possibility, two tables have a 1-M relationship to each other (two relationships, one 1-M going each way). A [bad] example of the more common situation would be if you had a job assignment database, where one table held one row for each employee and a job assignment, and another table held one row for each job with one of the assigned employees. Here, you would have multiple rows for each employee in the first table, one for each job assignment, and multiple rows for each job in the second table, one for each employee assigned to the project. These tables have a M-M: each employee in the first table has many job assignments from the second table, and each job has many employees assigned to it from the first table. This is the tip of the iceberg on this topic...see the links below for more information and see the diagram below for a simplified example of an E-R diagram.
|ABS(X)||Absolute value-converts negative numbers to positive, or leaves positive numbers alone|
|CEIL(X)||X is a decimal value that will be rounded up.|
|FLOOR(X)||X is a decimal value that will be rounded down.|
|GREATEST(X,Y)||Returns the largest of the two values.|
|LEAST(X,Y)||Returns the smallest of the two values.|
|MOD(X,Y)||Returns the remainder of X / Y.|
|POWER(X,Y)||Returns X to the power of Y.|
|ROUND(X,Y)||Rounds X to Y decimal places. If Y is omitted, X is rounded to the nearest integer.|
|SIGN(X)||Returns a minus if X < 0, else a plus.|
|SQRT(X)||Returns the square root of X.|
|LEFT(<string>,X)||Returns the leftmost X characters of the string.|
|RIGHT(<string>,X)||Returns the rightmost X characters of the string.|
|UPPER(<string>)||Converts the string to all uppercase letters.|
|LOWER(<string>)||Converts the string to all lowercase letters.|
|INITCAP(<string>)||Converts the string to initial caps.|
|LENGTH(<string>)||Returns the number of characters in the string.|
|<string>||<string>||Combines the two strings of text into one, concatenated string, where the first string is immediately followed by the second.|
|LPAD(<string>,X,'*')||Pads the string on the left with the * (or whatever character is inside the quotes), to make the string X characters long.|
|RPAD(<string>,X,'*')||Pads the string on the right with the * (or whatever character is inside the quotes), to make the string X characters long.|
|SUBSTR(<string>,X,Y)||Extracts Y letters from the string beginning at position X.|
|NVL(<column>,<value>)||The Null value function will substitute <value> for any NULLs for in the <column>. If the current value of <column> is not NULL, NVL has no effect.|
Here are the general forms of the statements discussed in this tutorial, plus some extra important ones (explanations given). REMEMBER that all of these statements may or may not be available on your system, so check documentation regarding availability:
ALTER TABLE <TABLE NAME> ADD|DROP|MODIFY (COLUMN SPECIFICATION[S]...see Create Table); --allows you to add or delete a column or columns from a table, or change the specification (data type, etc.) on an existing column; this statement is also used to change the physical specifications of a table (how a table is stored, etc.), but these definitions are DBMS-specific, so read the documentation. Also, these physical specifications are used with the Create Table statement, when a table is first created. In addition, only one option can be performed per Alter Table statement --either add, drop, OR modify in a single statement.
COMMIT; --makes changes made to some database systems permanent (since the last COMMIT; known as a transaction)
CREATE [UNIQUE] INDEX <INDEX NAME>
ON <TABLE NAME> (<COLUMN LIST>); --UNIQUE is optional; within brackets.
CREATE TABLE <TABLE NAME>
(<COLUMN NAME> <DATA TYPE> [(<SIZE>)] <COLUMN CONSTRAINT>,
...other columns); (also valid with ALTER TABLE)
--where SIZE is only used on certain data types (see above), and constraints include the following possibilities (automatically enforced by the DBMS; failure causes an error to be generated):
DELETE FROM <TABLE NAME> WHERE <CONDITION>;
INSERT INTO <TABLE NAME> [(<COLUMN
VALUES (<VALUE LIST>);
ROLLBACK; --Takes back any changes to the database that you have made, back to the last time you gave a Commit command...beware! Some software uses automatic committing on systems that use the transaction features, so the Rollback command may not work.
SELECT [DISTINCT|ALL] <LIST OF COLUMNS, FUNCTIONS, CONSTANTS,
FROM <LIST OF TABLES OR VIEWS>
[GROUP BY <GROUPING COLUMN(S)>]
[ORDER BY <ORDERING COLUMN(S)> [ASC|DESC]]; --where ASC|DESC allows the ordering to be done in ASCending or DESCending order
UPDATE <TABLE NAME>
SET <COLUMN NAME> = <VALUE>
[WHERE <CONDITION>]; --if the Where clause is left out, all rows will be updated according to the Set statement.
Using the example tables in the tutorial, write a SQL statement to:
1. Show each Antiques order and the last and first names of the person who ordered the item.
2. Show each column in the EmployeeStatisticsTable in alphabetical order by Position, then by EmployeeIDNo.
3. Show the annual budget for Benefits from the EmployeeStatisticsTable.
4. Using the IN Operator, show the names of the owners of Chairs.
5. Show the names of all Antiques Owners who have do not have an order placed.
6. Show the names of those who have placed Antique orders, with no duplicates (Hint: consider the order of tables in the From clause).
7. Delete all of Bob Smith's Antique orders (Hint: Bob's ID Number is 02).
8. Create an Antique order for a Rocking Chair for Jane Akins (Hint: Jane's ID Number is 21).
9. Create a table called Employees, with columns EmployeeIDNo (don't worry about trailing zeroes), FirstName, and LastName.
10. (Challenger) Show the annual budget for Salary by each position from the EmployeeStatisticsTable (Hint: Try GROUP BY).
11. What is the relationship between the AntiqueOwners table and the Owners table?
12. If you do not have a primary key in a table, the addition of what type of column is preferred to give the table a primary key?
13. Which function will allow you to substitute a given value for any Null values arising from a Select statement?
14. When using Embedded SQL, what do you need to create in order to iterate through the results of a multi-row query, one row at a time?
15. If all of the columns in all of the tables in your schema are dependent solely on the value of the primary key in each table, in which Normal Form is your design?
16. What term is used to describe the event of a database system automatically updating the values of foreign keys in other tables, when the value of a primary key is updated?
17. What database object provides fast access to the data in the rows of a table?
18. What type of SQL statement is used to change the attributes of a column?
19. In a Create Table statement, when a column is designated as NOT NULL, what does this mean?
20. If you wish to write a query that is based on other queries, rather than tables, what do these other queries need to be created as?
Answers (Queries may have more than one correct answer):
1. SELECT AntiqueOwners.OwnerLastName, AntiqueOwners.OwnerFirstName, Orders.ItemDesired
FROM AntiqueOwners, Orders
WHERE AntiqueOwners.OwnerID = Orders.OwnerID;
SELECT AntiqueOwners.OwnerLastName, AntiqueOwners.OwnerFirstName, Orders.ItemDesired
FROM AntiqueOwners RIGHT JOIN Orders ON AntiqueOwners.OwnerID = Orders.OwnerID;
2. SELECT *
ORDER BY Position, EmployeeIDNo;
3. SELECT Sum(Benefits)
4. SELECT OwnerLastName, OwnerFirstName
FROM AntiqueOwners, Antiques
WHERE Item In ('Chair')
AND AntiqueOwners.OwnerID = Antiques.BuyerID;
5. SELECT OwnerLastName, OwnerFirstName
WHERE OwnerID NOT IN
6. SELECT DISTINCT OwnerLastName, OwnerFirstName
FROM Orders, AntiqueOwners
WHERE AntiqueOwners.OwnerID = Orders.OwnerID;
or to use JOIN notation:
SELECT DISTINCT AntiqueOwners.OwnerLastName, AntiqueOwners.OwnerFirstName
FROM AntiqueOwners RIGHT JOIN Orders ON AntiqueOwners.OwnerID = Orders.OwnerID;
7. DELETE FROM ORDERS
WHERE OWNERID = 02;
8. INSERT INTO ORDERS VALUES (21, 'Rocking Chair');
9. CREATE TABLE EMPLOYEES
(EmployeeIDNo INTEGER NOT NULL,
FirstName CHAR(40) NOT NULL,
LastName CHAR(40) NOT NULL);
10. SELECT Position, Sum(Salary)
GROUP BY Position;
12. An integer identification number; an auto-increment ID is preferred.
14. A Cursor.
15. Third Normal Form.
16. Cascading update.
17. An Index.
18. ALTER TABLE.
19. A value is required in this column for every row in the table.
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I hope you have learned something from this introductory look at a very important language that is becoming more prevalent in the world of client-server computing. I wrote this web page in order to contribute something of value to the web and the Internet community. In fact, I have been informed that this document is being used at several colleges and companies for use in database classes, and as a resource for many others. In addition, I would like to thank all of the people from across five continents who have contacted me regarding this web page.
In addition, I strongly urge you to visit some of the database links shown above, especially if you're interested in advanced topics, such as the SQL-92 standard, different relational DBMS's, and advanced query processing. Unfortunately, however, the number of database web sites remains small, and if you are unable to find the information for which you are looking at the links displayed above, the information may not be available on the web, and that you will have to potentially contact a database vendor for the information you seek. In fact, if you're using a well-known, name-brand DBMS, the web site of your vendor is often the first and best place to look for information.
The PDF/Adobe Acrobat version of the tutorial is available directly from Matthew Kelly at Waterfront Communications (www.highcroft.com)...thanks again.
Copyright 1996-2000, James Hoffman. This document can be used for free by any Internet user, but cannot be included in another document, another web site or server, published in any other form, or mass produced in any way.
Last updated: 7-25-2000; fixed misspellings; updated links and recommendations; added questions and Cartesian join (over 3,100 Yahoo! SQL Club members, 4th overall in Computers & Internet clubs; 1st overall in Programming Languages...297,500 hits; 2-26-99 thru 6-30-99). Approaching the fifth year of service to the worldwide Internet community.