7.1. Overview

7.1. Overview
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+ 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();
+