Queries
The last
common SQL command,
SELECT
, enables you to view the data in the
database. This action is by far the most common action performed in
SQL. While data entry and modifications do happen on occasion, most
databases spend the vast majority of their lives serving up data for
reading. The general form of the SELECT
statement
is as follows:
SELECTcolumn1
,column2
,...
,columnN
FROMtable1
,table2
,...
,tableN
[WHEREclause
]
This syntax is certainly the most common way to retrieve data from
any SQL database. The SELECT
statement enables you
to identify the columns you want from one or more tables. The
WHERE
clause identifies the rows with the data you
seek.
Of course, there are variations for performing complex and powerful
queries. (We cover the full range of the SELECT
syntax in Chapter 15.) The simplest form
is:
SELECT 1;
This simple, though completely useless query returns a result set with a single row containing a single column with the value of 1. A more useful version of this query might be something like:
mysql> SELECT DATABASE( );
+------------+
| DATABASE( ) |
+------------+
| test |
+------------+
1 row in set (0.01 sec)
The expression DATABASE(
)
is a MySQL function that returns the
name of the current database. (We will cover functions in more detail
later in the chapter.) Nevertheless, you can see how simple SQL can
provide a quick-and-dirty way of finding out important information.
Most of the time, however, you should use slightly more complex queries that ...
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