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The process of retrieving or the command to retrieve data from a database is called aquery. In SQL theSELECT command is used to specify queries. The general syntax of theSELECT 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 calledtable1, this command would retrieve all rows and all user-defined columns fromtable1. (The method of retrieval depends on the client application. For example, thepsql 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, iftable1 has columns nameda,b, andc (and perhaps others) you can make the following query:

SELECT a, b + c FROM table1;

(assuming thatb andc are of a numerical data type). SeeSection 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 theSELECT 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();