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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 >Querying a Table</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="The SQL Language" HREF="tutorial-sql.html"><LINK REL="PREVIOUS" TITLE="Populating a Table With Rows" HREF="tutorial-populate.html"><LINK REL="NEXT" TITLE="Joins Between Tables" HREF="tutorial-join.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="Populating a Table With Rows" HREF="tutorial-populate.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="tutorial-sql.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 2. The <ACRONYM CLASS="ACRONYM" >SQL</ACRONYM > Language</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="Joins Between Tables" HREF="tutorial-join.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="TUTORIAL-SELECT" >2.5. Querying a Table</A ></H1 ><P > To retrieve data from a table, the table is <I CLASS="FIRSTTERM" >queried</I >. An <ACRONYM CLASS="ACRONYM" >SQL</ACRONYM > <TT CLASS="COMMAND" >SELECT</TT > statement is used to do this. The statement is divided into a select list (the part that lists the columns to be returned), a table list (the part that lists the tables from which to retrieve the data), and an optional qualification (the part that specifies any restrictions). For example, to retrieve all the rows of table <TT CLASS="STRUCTNAME" >weather</TT >, type: </P><PRE CLASS="PROGRAMLISTING" >SELECT * FROM weather;</PRE ><P> Here <TT CLASS="LITERAL" >*</TT > is a shorthand for <SPAN CLASS="QUOTE" >"all columns"</SPAN >. <A NAME="AEN721" HREF="#FTN.AEN721" ><SPAN CLASS="footnote" >[1]</SPAN ></A > So the same result would be had with: </P><PRE CLASS="PROGRAMLISTING" >SELECT city, temp_lo, temp_hi, prcp, date FROM weather;</PRE ><P> The output should be: </P><PRE CLASS="SCREEN" > city | temp_lo | temp_hi | prcp | date ---------------+---------+---------+------+------------ San Francisco | 46 | 50 | 0.25 | 1994-11-27 San Francisco | 43 | 57 | 0 | 1994-11-29 Hayward | 37 | 54 | | 1994-11-29 (3 rows)</PRE ><P> </P ><P > You can write expressions, not just simple column references, in the select list. For example, you can do: </P><PRE CLASS="PROGRAMLISTING" >SELECT city, (temp_hi+temp_lo)/2 AS temp_avg, date FROM weather;</PRE ><P> This should give: </P><PRE CLASS="SCREEN" > city | temp_avg | date ---------------+----------+------------ San Francisco | 48 | 1994-11-27 San Francisco | 50 | 1994-11-29 Hayward | 45 | 1994-11-29 (3 rows)</PRE ><P> Notice how the <TT CLASS="LITERAL" >AS</TT > clause is used to relabel the output column. (The <TT CLASS="LITERAL" >AS</TT > clause is optional.) </P ><P > A query can be <SPAN CLASS="QUOTE" >"qualified"</SPAN > by adding a <TT CLASS="LITERAL" >WHERE</TT > clause that specifies which rows are wanted. The <TT CLASS="LITERAL" >WHERE</TT > clause contains a Boolean (truth value) expression, and only rows for which the Boolean expression is true are returned. The usual Boolean operators (<TT CLASS="LITERAL" >AND</TT >, <TT CLASS="LITERAL" >OR</TT >, and <TT CLASS="LITERAL" >NOT</TT >) are allowed in the qualification. For example, the following retrieves the weather of San Francisco on rainy days: </P><PRE CLASS="PROGRAMLISTING" >SELECT * FROM weather WHERE city = 'San Francisco' AND prcp > 0.0;</PRE ><P> Result: </P><PRE CLASS="SCREEN" > city | temp_lo | temp_hi | prcp | date ---------------+---------+---------+------+------------ San Francisco | 46 | 50 | 0.25 | 1994-11-27 (1 row)</PRE ><P> </P ><P > You can request that the results of a query be returned in sorted order: </P><PRE CLASS="PROGRAMLISTING" >SELECT * FROM weather ORDER BY city;</PRE ><P> </P><PRE CLASS="SCREEN" > city | temp_lo | temp_hi | prcp | date ---------------+---------+---------+------+------------ Hayward | 37 | 54 | | 1994-11-29 San Francisco | 43 | 57 | 0 | 1994-11-29 San Francisco | 46 | 50 | 0.25 | 1994-11-27</PRE ><P> In this example, the sort order isn't fully specified, and so you might get the San Francisco rows in either order. But you'd always get the results shown above if you do: </P><PRE CLASS="PROGRAMLISTING" >SELECT * FROM weather ORDER BY city, temp_lo;</PRE ><P> </P ><P > You can request that duplicate rows be removed from the result of a query: </P><PRE CLASS="PROGRAMLISTING" >SELECT DISTINCT city FROM weather;</PRE ><P> </P><PRE CLASS="SCREEN" > city --------------- Hayward San Francisco (2 rows)</PRE ><P> Here again, the result row ordering might vary. You can ensure consistent results by using <TT CLASS="LITERAL" >DISTINCT</TT > and <TT CLASS="LITERAL" >ORDER BY</TT > together: <A NAME="AEN755" HREF="#FTN.AEN755" ><SPAN CLASS="footnote" >[2]</SPAN ></A > </P><PRE CLASS="PROGRAMLISTING" >SELECT DISTINCT city FROM weather ORDER BY city;</PRE ><P> </P ></DIV ><H3 CLASS="FOOTNOTES" >Notes</H3 ><TABLE BORDER="0" CLASS="FOOTNOTES" WIDTH="100%" ><TR ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%" ><A NAME="FTN.AEN721" HREF="tutorial-select.html#AEN721" ><SPAN CLASS="footnote" >[1]</SPAN ></A ></TD ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%" ><P > While <TT CLASS="LITERAL" >SELECT *</TT > is useful for off-the-cuff queries, it is widely considered bad style in production code, since adding a column to the table would change the results. </P ></TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%" ><A NAME="FTN.AEN755" HREF="tutorial-select.html#AEN755" ><SPAN CLASS="footnote" >[2]</SPAN ></A ></TD ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%" ><P > In some database systems, including older versions of <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN >, the implementation of <TT CLASS="LITERAL" >DISTINCT</TT > automatically orders the rows and so <TT CLASS="LITERAL" >ORDER BY</TT > is unnecessary. But this is not required by the SQL standard, and current <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > does not guarantee that <TT CLASS="LITERAL" >DISTINCT</TT > causes the rows to be ordered. </P ></TD ></TR ></TABLE ><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="tutorial-populate.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="tutorial-join.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Populating a Table With Rows</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="tutorial-sql.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Joins Between Tables</TD ></TR ></TABLE ></DIV ></BODY ></HTML >