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
[WITHwith_queries
] SELECTselect_list
FROMtable_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 7.3절 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();