Page 386 - Beginning PHP 5.3

Part III: Using PHP in Practice
So when creating indexes on a table, don ’ t create more than you need. Limit indexed columns to those
that will be searched or sorted frequently. If required, you can create additional indexes on a table as you
need them to increase performance.

Introducing SQL Statements
To actually work with databases and tables, you use SQL statements. Common statements include:

❑ SELECT — Retrieves data from one or more tables

❑ INSERT — Inserts data into a table

❑ REPLACE — Replaces data in a table. If the same record exists in the table, the statement
overwrites the record with the new data

❑ UPDATE — Updates data in a table

❑ DELETE — Deletes data from a table
Other often - used statements create or modify tables and databases themselves, rather than manipulating
the data stored in a table:
❑ CREATE — Creates a database, table or index

❑ ALTER — Modifies the structure of a table
❑ DROP — Wipes out a database or table

You learn about most of these statements as you work through the next few chapters. Just to give you a
taste though, let ’ s take a look at the typical form of a MySQL SELECT statement, which retrieves records
from a table. Operations performed with SELECT are known as queries (hence the name “ Structured
Query Language ” ):

SELECT field1 , field2 , ... , fieldn FROM table WHERE condition
A statement may expand to multiple lines. Here ’ s a simple example of a real multi - line SQL statement:

SELECT lastName, firstName
FROM users

WHERE firstName = ‘John’
Take a closer look at the FROM and WHERE clauses in the query. The query returns any record from the
.

users table where the value of the firstName field is “ John ” Assuming there actually is a table
called users in the database, the query ’ s output might look like this:
Simpleton John
Smith John
Thomas John

The returned values are known as the result set . As you see later, you can loop through all the rows in a
result set within your PHP script. If your query finds no rows, NULL (discussed in the next section) is
returned instead.

Other SQL statements such as DELETE or INSERT don ’ t return a result set.

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