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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 >Combining Queries</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="Select Lists" HREF="queries-select-lists.html"><LINK REL="NEXT" TITLE="Sorting Rows" HREF="queries-order.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="Select Lists" HREF="queries-select-lists.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="Sorting Rows" HREF="queries-order.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="QUERIES-UNION" >7.4. Combining Queries</A ></H1 ><P > The results of two queries can be combined using the set operations union, intersection, and difference. The syntax is </P><PRE CLASS="SYNOPSIS" ><TT CLASS="REPLACEABLE" ><I >query1</I ></TT > UNION [<SPAN CLASS="OPTIONAL" >ALL</SPAN >] <TT CLASS="REPLACEABLE" ><I >query2</I ></TT > <TT CLASS="REPLACEABLE" ><I >query1</I ></TT > INTERSECT [<SPAN CLASS="OPTIONAL" >ALL</SPAN >] <TT CLASS="REPLACEABLE" ><I >query2</I ></TT > <TT CLASS="REPLACEABLE" ><I >query1</I ></TT > EXCEPT [<SPAN CLASS="OPTIONAL" >ALL</SPAN >] <TT CLASS="REPLACEABLE" ><I >query2</I ></TT ></PRE ><P> <TT CLASS="REPLACEABLE" ><I >query1</I ></TT > and <TT CLASS="REPLACEABLE" ><I >query2</I ></TT > are queries that can use any of the features discussed up to this point. Set operations can also be nested and chained, for example </P><PRE CLASS="SYNOPSIS" ><TT CLASS="REPLACEABLE" ><I >query1</I ></TT > UNION <TT CLASS="REPLACEABLE" ><I >query2</I ></TT > UNION <TT CLASS="REPLACEABLE" ><I >query3</I ></TT ></PRE ><P> which is executed as: </P><PRE CLASS="SYNOPSIS" >(<TT CLASS="REPLACEABLE" ><I >query1</I ></TT > UNION <TT CLASS="REPLACEABLE" ><I >query2</I ></TT >) UNION <TT CLASS="REPLACEABLE" ><I >query3</I ></TT ></PRE ><P> </P ><P > <TT CLASS="LITERAL" >UNION</TT > effectively appends the result of <TT CLASS="REPLACEABLE" ><I >query2</I ></TT > to the result of <TT CLASS="REPLACEABLE" ><I >query1</I ></TT > (although there is no guarantee that this is the order in which the rows are actually returned). Furthermore, it eliminates duplicate rows from its result, in the same way as <TT CLASS="LITERAL" >DISTINCT</TT >, unless <TT CLASS="LITERAL" >UNION ALL</TT > is used. </P ><P > <TT CLASS="LITERAL" >INTERSECT</TT > returns all rows that are both in the result of <TT CLASS="REPLACEABLE" ><I >query1</I ></TT > and in the result of <TT CLASS="REPLACEABLE" ><I >query2</I ></TT >. Duplicate rows are eliminated unless <TT CLASS="LITERAL" >INTERSECT ALL</TT > is used. </P ><P > <TT CLASS="LITERAL" >EXCEPT</TT > returns all rows that are in the result of <TT CLASS="REPLACEABLE" ><I >query1</I ></TT > but not in the result of <TT CLASS="REPLACEABLE" ><I >query2</I ></TT >. (This is sometimes called the <I CLASS="FIRSTTERM" >difference</I > between two queries.) Again, duplicates are eliminated unless <TT CLASS="LITERAL" >EXCEPT ALL</TT > is used. </P ><P > In order to calculate the union, intersection, or difference of two queries, the two queries must be <SPAN CLASS="QUOTE" >"union compatible"</SPAN >, which means that they return the same number of columns and the corresponding columns have compatible data types, as described in <A HREF="typeconv-union-case.html" >Section 10.5</A >. </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-select-lists.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-order.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Select Lists</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="queries.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Sorting Rows</TD ></TR ></TABLE ></DIV ></BODY ></HTML >