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Add FAQ entry to mention using COALESCE() for concatenation of possible

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NULLs.
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Bruce Momjian
2006-12-11 22:44:53 +00:00
parent 9fa12ddda6
commit 051b52c886
2 changed files with 30 additions and 11 deletions
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doc/src/FAQ/FAQ.html
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@ -10,7 +10,7 @@
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Tue Dec 5 18:13:32 EST 2006</P>
<P>Last updated: Mon Dec 11 17:44:33 EST 2006</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:bruce@momjian.us">bruce@momjian.us</A>)
@ -86,8 +86,8 @@
searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?<BR>
<A href="#item4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
NULL</SMALL> or not?<BR>
is <SMALL>NULL</SMALL>? How do I concatenate possible <SMALL>NULL</SMALL>s?
How can I sort on whether a field is <SMALL> NULL</SMALL> or not?<BR>
<A href="#item4.10">4.10</A>) What is the difference between the
various character types?<BR>
<A href="#item4.11.1">4.11.1</A>) How do I create a
@ -823,10 +823,14 @@ table?</TD><TD>unlimited</TD></TR>
identical values that differ only in case. To force a particular
case to be stored in the column, use a <SMALL>CHECK</SMALL>
constraint or a trigger.</P>
<A href="#item4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>? How do I concatenate possible <SMALL>NULL</SMALL>s?
How can I sort on whether a field is <SMALL> NULL</SMALL> or not?<BR>
<H3 id="item4.9">4.9) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
NULL</SMALL> or not?</H3>
is <SMALL>NULL</SMALL>? How do I concatenate possible <SMALL>NULL</SMALL>s?
How can I sort on whether a field is <SMALL> NULL</SMALL> or not?</H3>
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
NOT NULL</SMALL>, like this:</P>
@ -837,6 +841,13 @@ table?</TD><TD>unlimited</TD></TR>
WHERE col IS NULL;
</PRE>
<P>To concatentate with possible <SMALL>NULL</SMALL>s, use <I>COALESCE()</I>,
like this:</P>
<PRE>
SELECT COALESCE(col1, '') || COALESCE(col2, '')
FROM tab
</PRE>
<P>To sort by the <SMALL>NULL</SMALL> status, use the <SMALL>IS NULL</SMALL>
and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>ORDER BY</SMALL> clause.
Things that are <I>true</I> will sort higher than things that are <I>false</I>,