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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Overview</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.2.24 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Queries" HREF="queries.html"><LINK REL="PREVIOUS" TITLE="Queries" HREF="queries.html"><LINK REL="NEXT" TITLE="Table Expressions" HREF="queries-table-expressions.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2017-11-06T22:43:11"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.2.24 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Queries" HREF="queries.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="queries.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 7. Queries</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="Table Expressions" HREF="queries-table-expressions.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="QUERIES-OVERVIEW" >7.1. Overview</A ></H1 ><P > The process of retrieving or the command to retrieve data from a database is called a <I CLASS="FIRSTTERM" >query</I >. In SQL the <A HREF="sql-select.html" >SELECT</A > command is used to specify queries. The general syntax of the <TT CLASS="COMMAND" >SELECT</TT > command is </P><PRE CLASS="SYNOPSIS" >[<SPAN CLASS="OPTIONAL" >WITH <TT CLASS="REPLACEABLE" ><I >with_queries</I ></TT ></SPAN >] SELECT <TT CLASS="REPLACEABLE" ><I >select_list</I ></TT > FROM <TT CLASS="REPLACEABLE" ><I >table_expression</I ></TT > [<SPAN CLASS="OPTIONAL" ><TT CLASS="REPLACEABLE" ><I >sort_specification</I ></TT ></SPAN >]</PRE ><P> The following sections describe the details of the select list, the table expression, and the sort specification. <TT CLASS="LITERAL" >WITH</TT > queries are treated last since they are an advanced feature. </P ><P > A simple kind of query has the form: </P><PRE CLASS="PROGRAMLISTING" >SELECT * FROM table1;</PRE ><P> Assuming that there is a table called <TT CLASS="LITERAL" >table1</TT >, this command would retrieve all rows and all user-defined columns from <TT CLASS="LITERAL" >table1</TT >. (The method of retrieval depends on the client application. For example, the <SPAN CLASS="APPLICATION" >psql</SPAN > 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 <TT CLASS="LITERAL" >*</TT > 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 <TT CLASS="LITERAL" >table1</TT > has columns named <TT CLASS="LITERAL" >a</TT >, <TT CLASS="LITERAL" >b</TT >, and <TT CLASS="LITERAL" >c</TT > (and perhaps others) you can make the following query: </P><PRE CLASS="PROGRAMLISTING" >SELECT a, b + c FROM table1;</PRE ><P> (assuming that <TT CLASS="LITERAL" >b</TT > and <TT CLASS="LITERAL" >c</TT > are of a numerical data type). See <A HREF="queries-select-lists.html" >Section 7.3</A > for more details. </P ><P > <TT CLASS="LITERAL" >FROM table1</TT > 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 <TT CLASS="COMMAND" >SELECT</TT > command as a calculator: </P><PRE CLASS="PROGRAMLISTING" >SELECT 3 * 4;</PRE ><P> This is more useful if the expressions in the select list return varying results. For example, you could call a function this way: </P><PRE CLASS="PROGRAMLISTING" >SELECT random();</PRE ><P> </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="queries.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="queries-table-expressions.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Queries</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="queries.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Table Expressions</TD ></TR ></TABLE ></DIV ></BODY ></HTML >