| Server IP : 103.119.228.120 / Your IP : 3.129.216.15 Web Server : Apache System : Linux v8.techscape8.com 3.10.0-1160.119.1.el7.tuxcare.els2.x86_64 #1 SMP Mon Jul 15 12:09:18 UTC 2024 x86_64 User : nobody ( 99) PHP Version : 5.6.40 Disable Function : shell_exec,symlink,system,exec,proc_get_status,proc_nice,proc_terminate,define_syslog_variables,syslog,openlog,closelog,escapeshellcmd,passthru,ocinum cols,ini_alter,leak,listen,chgrp,apache_note,apache_setenv,debugger_on,debugger_off,ftp_exec,dl,dll,myshellexec,proc_open,socket_bind,proc_close,escapeshellarg,parse_ini_filepopen,fpassthru,exec,passthru,escapeshellarg,escapeshellcmd,proc_close,proc_open,ini_alter,popen,show_source,proc_nice,proc_terminate,proc_get_status,proc_close,pfsockopen,leak,apache_child_terminate,posix_kill,posix_mkfifo,posix_setpgid,posix_setsid,posix_setuid,dl,symlink,shell_exec,system,dl,passthru,escapeshellarg,escapeshellcmd,myshellexec,c99_buff_prepare,c99_sess_put,fpassthru,getdisfunc,fx29exec,fx29exec2,is_windows,disp_freespace,fx29sh_getupdate,fx29_buff_prepare,fx29_sess_put,fx29shexit,fx29fsearch,fx29ftpbrutecheck,fx29sh_tools,fx29sh_about,milw0rm,imagez,sh_name,myshellexec,checkproxyhost,dosyayicek,c99_buff_prepare,c99_sess_put,c99getsource,c99sh_getupdate,c99fsearch,c99shexit,view_perms,posix_getpwuid,posix_getgrgid,posix_kill,parse_perms,parsesort,view_perms_color,set_encoder_input,ls_setcheckboxall,ls_reverse_all,rsg_read,rsg_glob,selfURL,dispsecinfo,unix2DosTime,addFile,system,get_users,view_size,DirFiles,DirFilesWide,DirPrintHTMLHeaders,GetFilesTotal,GetTitles,GetTimeTotal,GetMatchesCount,GetFileMatchesCount,GetResultFiles,fs_copy_dir,fs_copy_obj,fs_move_dir,fs_move_obj,fs_rmdir,SearchText,getmicrotime MySQL : ON | cURL : ON | WGET : ON | Perl : ON | Python : ON | Sudo : ON | Pkexec : ON Directory : /usr/local/ssl/share/doc/postgresql-9.2.24/html/ |
Upload File : |
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Aggregate Functions</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="Joins Between Tables"
HREF="tutorial-join.html"><LINK
REL="NEXT"
TITLE="Updates"
HREF="tutorial-update.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="Joins Between Tables"
HREF="tutorial-join.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="Updates"
HREF="tutorial-update.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="TUTORIAL-AGG"
>2.7. Aggregate Functions</A
></H1
><P
> Like most other relational database products,
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> supports
<I
CLASS="FIRSTTERM"
>aggregate functions</I
>.
An aggregate function computes a single result from multiple input rows.
For example, there are aggregates to compute the
<CODE
CLASS="FUNCTION"
>count</CODE
>, <CODE
CLASS="FUNCTION"
>sum</CODE
>,
<CODE
CLASS="FUNCTION"
>avg</CODE
> (average), <CODE
CLASS="FUNCTION"
>max</CODE
> (maximum) and
<CODE
CLASS="FUNCTION"
>min</CODE
> (minimum) over a set of rows.
</P
><P
> As an example, we can find the highest low-temperature reading anywhere
with:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT max(temp_lo) FROM weather;</PRE
><P>
</P><PRE
CLASS="SCREEN"
> max
-----
46
(1 row)</PRE
><P>
</P
><P
>
If we wanted to know what city (or cities) that reading occurred in,
we might try:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT city FROM weather WHERE temp_lo = max(temp_lo); <I
CLASS="LINEANNOTATION"
>WRONG</I
></PRE
><P>
but this will not work since the aggregate
<CODE
CLASS="FUNCTION"
>max</CODE
> cannot be used in the
<TT
CLASS="LITERAL"
>WHERE</TT
> clause. (This restriction exists because
the <TT
CLASS="LITERAL"
>WHERE</TT
> clause determines which rows will be
included in the aggregate calculation; so obviously it has to be evaluated
before aggregate functions are computed.)
However, as is often the case
the query can be restated to accomplish the desired result, here
by using a <I
CLASS="FIRSTTERM"
>subquery</I
>:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT city FROM weather
WHERE temp_lo = (SELECT max(temp_lo) FROM weather);</PRE
><P>
</P><PRE
CLASS="SCREEN"
> city
---------------
San Francisco
(1 row)</PRE
><P>
This is OK because the subquery is an independent computation
that computes its own aggregate separately from what is happening
in the outer query.
</P
><P
>
Aggregates are also very useful in combination with <TT
CLASS="LITERAL"
>GROUP
BY</TT
> clauses. For example, we can get the maximum low
temperature observed in each city with:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT city, max(temp_lo)
FROM weather
GROUP BY city;</PRE
><P>
</P><PRE
CLASS="SCREEN"
> city | max
---------------+-----
Hayward | 37
San Francisco | 46
(2 rows)</PRE
><P>
which gives us one output row per city. Each aggregate result is
computed over the table rows matching that city.
We can filter these grouped
rows using <TT
CLASS="LITERAL"
>HAVING</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT city, max(temp_lo)
FROM weather
GROUP BY city
HAVING max(temp_lo) < 40;</PRE
><P>
</P><PRE
CLASS="SCREEN"
> city | max
---------+-----
Hayward | 37
(1 row)</PRE
><P>
which gives us the same results for only the cities that have all
<TT
CLASS="STRUCTFIELD"
>temp_lo</TT
> values below 40. Finally, if we only care about
cities whose
names begin with <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>S</TT
>"</SPAN
>, we might do:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT city, max(temp_lo)
FROM weather
WHERE city LIKE 'S%'<A
NAME="CO.TUTORIAL-AGG-LIKE"
><B
>(1)</B
></A
>
GROUP BY city
HAVING max(temp_lo) < 40;</PRE
><P>
<DIV
CLASS="CALLOUTLIST"
><DL
COMPACT="COMPACT"
><DT
><A
HREF="tutorial-agg.html#CO.TUTORIAL-AGG-LIKE"
><B
>(1)</B
></A
></DT
><DD
> The <TT
CLASS="LITERAL"
>LIKE</TT
> operator does pattern matching and
is explained in <A
HREF="functions-matching.html"
>Section 9.7</A
>.
</DD
></DL
></DIV
>
</P
><P
> It is important to understand the interaction between aggregates and
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>'s <TT
CLASS="LITERAL"
>WHERE</TT
> and <TT
CLASS="LITERAL"
>HAVING</TT
> clauses.
The fundamental difference between <TT
CLASS="LITERAL"
>WHERE</TT
> and
<TT
CLASS="LITERAL"
>HAVING</TT
> is this: <TT
CLASS="LITERAL"
>WHERE</TT
> selects
input rows before groups and aggregates are computed (thus, it controls
which rows go into the aggregate computation), whereas
<TT
CLASS="LITERAL"
>HAVING</TT
> selects group rows after groups and
aggregates are computed. Thus, the
<TT
CLASS="LITERAL"
>WHERE</TT
> clause must not contain aggregate functions;
it makes no sense to try to use an aggregate to determine which rows
will be inputs to the aggregates. On the other hand, the
<TT
CLASS="LITERAL"
>HAVING</TT
> clause always contains aggregate functions.
(Strictly speaking, you are allowed to write a <TT
CLASS="LITERAL"
>HAVING</TT
>
clause that doesn't use aggregates, but it's seldom useful. The same
condition could be used more efficiently at the <TT
CLASS="LITERAL"
>WHERE</TT
>
stage.)
</P
><P
> In the previous example, we can apply the city name restriction in
<TT
CLASS="LITERAL"
>WHERE</TT
>, since it needs no aggregate. This is
more efficient than adding the restriction to <TT
CLASS="LITERAL"
>HAVING</TT
>,
because we avoid doing the grouping and aggregate calculations
for all rows that fail the <TT
CLASS="LITERAL"
>WHERE</TT
> check.
</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="tutorial-join.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-update.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Joins Between Tables</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"
>Updates</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>