The previous chapters explained how to create tables, how to fill
them with data, and how to manipulate that data. Now we finally
discuss how to retrieve the data out of the database.
The process of retrieving or the command to retrieve data from a
database is called a query. In SQL the
SELECT command is used to specify queries. The
general syntax of the SELECT command is
SELECT select_list FROM table_expression [sort_specification]
The following sections describe the details of the select list, the
table expression, and the sort specification.
The simplest kind of query has the form
SELECT * FROM table1;
Assuming that there is a table called table1,
this command would retrieve all rows and all columns from
table1. (The method of retrieval depends on the
client application. For example, the
psql program will display an ASCII-art
table on the screen, while client libraries will offer functions to
retrieve individual rows and columns.) The select list
specification * means all columns that the table
expression happens to provide. A select list can also select a
subset of the available columns or make calculations using the
columns. For example, if
table1 has columns named a,
b, and c (and perhaps others) you can make
the following query:
SELECT a, b + c FROM table1;
(assuming that b and c are of a numerical
data type).
See Section 4.3 for more details.
FROM table1 is a particularly simple kind of
table expression: it reads just one table. In general, table
expressions can be complex constructs of base tables, joins, and
subqueries. But you can also omit the table expression entirely and
use the SELECT command as a calculator:
SELECT 3 * 4;
This is more useful if the expressions in the select list return
varying results. For example, you could call a function this way:
SELECT random();
