You might want to get a listing of all heroes in your database whose last name begins with an E, or perhaps somebody parked a Yak Dirigible in your parking space and need to know who the driver is. You may also want your list sorted by special power or list only vehicles. All these (admittedly contrived) examples involve grabbing a subset of the original data. The SELECT statement is the SQL workhorse. 1. Click the SQL tab to get a query screen in phpMyAdmin. 2. Type in a query. 3. Click the Go button to see the query results. You’ve seen the simplest form of this command getting all the data in a table, like this: SELECT * FROM hero; Figure 9.19 shows this form of the SELECT statement operating on the hero table. phpMyAdmin is a wonderful tool for experimenting with SELECT statements because you can write the actual SQL by hand and see immediate results in a very clean format. If you don’t want to (or cannot) use phpMyAdmin, do the same exper- iments directly in MySQL. It will work, but the results are formatted as text and not always as easy to see. TRICK 323 C h a p t e r 9U s i n g M y S Q L t o C r e a t e D a t a b a s e s FIGURE 9.19 The SELECT query is in the top section and the results are shown underneath. The SELECT statement is extremely powerful because it can grab a subset of data that can return only the requested fields and records. This process of asking ques- tions of the database is commonly called a query . Note that phpMyAdmin some- times adds elements to the query (notably the limit information). This increases the query’s efficiency, but doesn’t substantially change the query. Limiting Columns You might not want all of the fields in a table. For example, you might just want a list of the name and weapon of everyone on your list. You can specify this by using the following SELECT statement, which is illustrated in Figure 9.20: SELECT name, weapon FROM hero; This may seem like a silly capability for such a simple database as the hero list. However, but you often run into extremely complicated tables with many fields and need to filter only a few fields. For example, I use a database to track student advisees. Each student’s information contains lots of data, but I might just want a list of names and e-mail addresses. The ability to isolate the fields I need is one way to get useful information from a database. The results of a query look a lot like a new table. You can think of a query result as a temporary table. 324 P H P 5 /M y S Q L P r o g r a m m i n g f o r t h e A b s o l u t e B e g i n n e r FIGURE 9.20 This query returns only the names and weapons. Limiting Rows with the WHERE Clause In addition to limiting the columns returned in a query, you may be interested in limiting the number of rows. For example, you might run across an evil villain who can only be defeated by a laser pointer. The query shown in Figure 9.21 illus- trates a query that solves exactly this dilemma. This code returns only the rows matching a specific condition: SELECT * FROM hero WHERE weapon = ‘Laser Pointer’; Adding a Condition with a WHERE Clause A WHERE statement in a query specifies which row(s) you want to see. This clause allows you to specify a condition. The database manager checks every record in the table. If the condition is TRUE for that record, it is included in the result set. The conditions in a WHERE clause are similar to those in PHP code, but they are not exactly the same. Use these symbols in SQL: • For equality use the single equal sign ( =). • Encase text elements in single quotation marks ( ‘). • Use <, >, and <= or >= and != conditions to limit your search. 325 C h a p t e r 9U s i n g M y S Q L t o C r e a t e D a t a b a s e s FIGURE 9.21 If you know how to set up the query, you can get very specific results. In this case, the query selects only those heroes with a laser pointer. Comparison operators are easy to understand for numeric data, such as integers and real numbers. It’s not quite so obvious how a language will treat text compar- isons. SQL has developed some standard rules, but each implementation might be somewhat different. SQL generally works in a case-insensitive way, so Yak-Bot would match yak-bot or yAK-bOT. Also, the < and > operators refer to alphabetic order, so the following selects all the records where the hero’s name starts with A, B, or C. SELECT * FROM hero WHERE name < ‘D’; Using the LIKE Clause for Partial Matches Often you do not know the exact value of a field you are trying to match. The LIKE clause allows you to specify partial matches. For example, which heroes have some sort of super power? This query returns each hero whose power begins with the value Super: SELECT * FROM hero WHERE power LIKE ‘Super%’; The percent sign (%) can be a wild card, which indicates any character, any num- ber of times. You can use a variation of the LIKE clause to find information about all heroes with a transportation scheme that starts with the letter B: SELECT name, transportation FROM hero WHERE transportation LIKE ‘B%’; You can also use the underscore character (_) to specify one character. The simple wildcard character support in SQL is sufficient for many purposes. If you like regular expressions, you can use the REGEXP clause to specify whether a field matches a regular expression. This is a very powerful tool, but it is an extension to the SQL standard. It works fine in MySQL, but it is not supported in all SQL databases. Generating Multiple Conditions You can combine conditions with AND, OR, and NOT keywords for more-complex expressions. For example, the following code selects those heroes whose trans- portation starts with B and who have a power with super in its name. TRICK TRICK 326 P H P 5 /M y S Q L P r o g r a m m i n g f o r t h e A b s o l u t e B e g i n n e r SELECT * FROM hero WHERE transportation LIKE ‘B%’ AND power LIKE ‘%super%’; Creating compound expressions is very useful as you build more-complex data- bases with multiple tables. Sorting Results with the ORDER BY Clause One more nifty SELECT statement feature is the ability to sort results by any field. Figures 9.22 and 9.23 illustrate how the ORDER BY clause can determine how tables are sorted. The ORDER BY clause allows you to determine how the data is sorted. You can spec- ify any field you wish as the sorting field. As you can see in Figure 9.23, the DESC clause specifies that data should be sorted in descending order. Changing Data with the UPDATE Statement You can use SQL to modify the data in a database. The key to this behavior is the UPDATE statement. An example helps it make sense: UPDATE hero SET power = ‘Super Electric Toe’ WHERE name = ‘Lightning Guardian’; 327 C h a p t e r 9U s i n g M y S Q L t o C r e a t e D a t a b a s e s FIGURE 9.22 This query shows the entire database sorted by the weapon name. . data. The SELECT statement is the SQL workhorse. 1. Click the SQL tab to get a query screen in phpMyAdmin. 2. Type in a query. 3. Click the Go button to see the query results. You’ve seen. this: SELECT * FROM hero; Figure 9.19 shows this form of the SELECT statement operating on the hero table. phpMyAdmin is a wonderful tool for experimenting with SELECT statements because you can write the. by hand and see immediate results in a very clean format. If you don’t want to (or cannot) use phpMyAdmin, do the same exper- iments directly in MySQL. It will work, but the results are formatted