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
[WITH with_queries] 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.
WITH
queries are treated last since they are an advanced feature.
A simple 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 user-defined 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 extract individual values from the query result.) 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 7.3 for more details.
FROM table1
is a 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();