<HTML ><HEAD ><TITLE >Queries</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.73 "><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 7.3.2 Documentation" HREF="index.html"><LINK REL="UP" TITLE="PostgreSQL 7.3.2 User's Guide" HREF="user.html"><LINK REL="PREVIOUS" TITLE="Deleting Data" HREF="dml-delete.html"><LINK REL="NEXT" TITLE="Table Expressions" HREF="queries-table-expressions.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META NAME="creation" CONTENT="2003-02-03T20:17:34"></HEAD ><BODY CLASS="CHAPTER" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >PostgreSQL 7.3.2 Documentation</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="dml-delete.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="queries-table-expressions.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="CHAPTER" ><H1 ><A NAME="QUERIES" >Chapter 4. Queries</A ></H1 ><DIV CLASS="TOC" ><DL ><DT ><B >Table of Contents</B ></DT ><DT >4.1. <A HREF="queries.html#QUERIES-OVERVIEW" >Overview</A ></DT ><DT >4.2. <A HREF="queries-table-expressions.html" >Table Expressions</A ></DT ><DD ><DL ><DT >4.2.1. <A HREF="queries-table-expressions.html#QUERIES-FROM" >The FROM Clause</A ></DT ><DT >4.2.2. <A HREF="queries-table-expressions.html#QUERIES-WHERE" >The WHERE Clause</A ></DT ><DT >4.2.3. <A HREF="queries-table-expressions.html#QUERIES-GROUP" >The GROUP BY and HAVING Clauses</A ></DT ></DL ></DD ><DT >4.3. <A HREF="queries-select-lists.html" >Select Lists</A ></DT ><DD ><DL ><DT >4.3.1. <A HREF="queries-select-lists.html#QUERIES-SELECT-LIST-ITEMS" >Select-List Items</A ></DT ><DT >4.3.2. <A HREF="queries-select-lists.html#QUERIES-COLUMN-LABELS" >Column Labels</A ></DT ><DT >4.3.3. <A HREF="queries-select-lists.html#QUERIES-DISTINCT" >DISTINCT</A ></DT ></DL ></DD ><DT >4.4. <A HREF="queries-union.html" >Combining Queries</A ></DT ><DT >4.5. <A HREF="queries-order.html" >Sorting Rows</A ></DT ><DT >4.6. <A HREF="queries-limit.html" >LIMIT and OFFSET</A ></DT ></DL ></DIV ><P > The previous chapters explained how to create tables, how to fill them with data, and how to manipulate that data. Now we finally discuss how to retrieve the data out of the database. </P ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="QUERIES-OVERVIEW" >4.1. 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 <TT CLASS="COMMAND" >SELECT</TT > command is used to specify queries. The general syntax of the <TT CLASS="COMMAND" >SELECT</TT > command is </P><PRE CLASS="SYNOPSIS" >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. </P ><P > The simplest 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 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 retrieve individual rows and columns.) 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 4.3</A > for more details. </P ><P > <TT CLASS="LITERAL" >FROM table1</TT > is a particularly 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 ><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="dml-delete.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" >Deleting Data</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="user.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Table Expressions</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
