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 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();
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