From 051b52c886c93d344db7da44a3ce99105a433713 Mon Sep 17 00:00:00 2001
From: Bruce Momjian <bruce@momjian.us>
Date: Mon, 11 Dec 2006 22:44:53 +0000
Subject: [PATCH] Add FAQ entry to mention using COALESCE() for concatenation
of possible NULLs.
---
doc/FAQ | 18 +++++++++++++-----
doc/src/FAQ/FAQ.html | 23 +++++++++++++++++------
2 files changed, 30 insertions(+), 11 deletions(-)
diff --git a/doc/FAQ b/doc/FAQ
index 69351486443..2939d754411 100644
--- a/doc/FAQ
+++ b/doc/FAQ
@@ -1,7 +1,7 @@
Frequently Asked Questions (FAQ) for PostgreSQL
- Last updated: Tue Dec 5 18:13:32 EST 2006
+ Last updated: Mon Dec 11 17:44:33 EST 2006
Current maintainer: Bruce Momjian (bruce@momjian.us)
@@ -59,8 +59,9 @@
4.8) How do I perform regular expression searches and case-insensitive
regular expression searches? How do I use an index for
case-insensitive searches?
- 4.9) In a query, how do I detect if a field is NULL? How can I sort on
- whether a field is NULL or not?
+ 4.9) In a query, how do I detect if a field is NULL? How do I
+ concatenate possible NULLs? How can I sort on whether a field is NULL
+ or not?
4.10) What is the difference between the various character types?
4.11.1) How do I create a serial/auto-incrementing field?
4.11.2) How do I get the value of a SERIAL insert?
@@ -631,15 +632,22 @@
upper and lowercase characters, it can not have identical values that
differ only in case. To force a particular case to be stored in the
column, use a CHECK constraint or a trigger.
+ 4.9) In a query, how do I detect if a field is NULL? How do I
+ concatenate possible NULLs? How can I sort on whether a field is NULL
+ or not?
- 4.9) In a query, how do I detect if a field is NULL? How can I sort on
- whether a field is NULL or not?
+ 4.9) In a query, how do I detect if a field is NULL? How do I concatenate
+ possible NULLs? How can I sort on whether a field is NULL or not?
You test the column with IS NULL and IS NOT NULL, like this:
SELECT *
FROM tab
WHERE col IS NULL;
+ To concatentate with possible NULLs, use COALESCE(), like this:
+ SELECT COALESCE(col1, '') || COALESCE(col2, '')
+ FROM tab
+
To sort by the NULL status, use the IS NULL and IS NOT NULL modifiers
in your ORDER BY clause. Things that are true will sort higher than
things that are false, so the following will put NULL entries at the
diff --git a/doc/src/FAQ/FAQ.html b/doc/src/FAQ/FAQ.html
index ca960bb888f..2a259db3d05 100644
--- a/doc/src/FAQ/FAQ.html
+++ b/doc/src/FAQ/FAQ.html
@@ -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>,
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