-<!-- $Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.51 2001/02/12 14:21:27 momjian Exp $ -->
+<!-- $Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.52 2001/02/18 17:17:17 petere Exp $ -->
<chapter id="functions">
<title>Functions and Operators</title>
If you are concerned about portability then take note that most of
the functions and operators described in this chapter, with the
exception of the most trivial arithmetic and comparison operators
- and some explicitly marked functions, are not specified by the SQL
- standard. However, many other RDBMS packages provide a lot of the
+ and some explicitly marked functions, are not specified by the <acronym>SQL</acronym>
+ standard. However, many other <acronym>RDBMS</acronym> packages provide a lot of the
same or similar functions, and some of the ones provided in
<productname>Postgres</productname> have in fact been inspired by
other implementations.
<member>NOT</member>
</simplelist>
- SQL uses a three-valued boolean logic where NULL represents
+ <acronym>SQL</acronym> uses a three-valued Boolean logic where NULL represents
<quote>unknown</quote>. Observe the following truth tables:
<informaltable>
<para>
Comparison operators are available for all data types where this
makes sense. All comparison operators are binary operators that
- return boolean values; expressions like <literal>1 < 2 <
- 3</literal> are not valid (because there is no
- <literal><</literal> operator to compare a boolean with
+ return values of type <type>boolean</type>; expressions like
+ <literal>1 < 2 < 3</literal> are not valid (because there is
+ no <literal><</literal> operator to compare a Boolean value with
<literal>3</literal>).
</para>
</para>
<para>
- SQL defines some string functions with a special syntax where
+ <acronym>SQL</acronym> defines some string functions with a special syntax where
certain keywords rather than commas are used to separate the
arguments. Details are in <xref linkend="functions-string-sql">.
These functions are also implemented using the regular syntax for
<para>
Additional string manipulation functions are available and are
listed below. Some of them are used internally to implement the
- SQL string functions listed above.
+ <acronym>SQL</acronym> string functions listed above.
</para>
<table id="functions-string-other">
<para>
The keyword <token>ILIKE</token> can be used instead of
<token>LIKE</token> to make the match case insensitive according
- to the active locale. This is not in the SQL standard but is a
+ to the active locale. This is not in the <acronym>SQL</acronym> standard but is a
<productname>Postgres</productname> extension.
</para>
<sect2 id="functions-regexp">
- <title>POSIX Regular Expressions</title>
+ <title><acronym>POSIX</acronym> Regular Expressions</title>
<table>
<title>Regular Expression Match Operators</title>
</table>
<para>
- POSIX regular expressions provide a more powerful means for
+ <acronym>POSIX</acronym> regular expressions provide a more powerful means for
pattern matching than the <function>LIKE</function> function.
Many Unix tools such as <command>egrep</command>,
<command>sed</command>, or <command>awk</command> use a pattern
<!-- derived from the re_format.7 man page -->
<para>
- Regular expressions (<quote>RE</quote>s), as defined in POSIX
+ Regular expressions (<quote>RE</quote>s), as defined in <acronym>POSIX</acronym>
1003.2, come in two forms: modern REs (roughly those of
<command>egrep</command>; 1003.2 calls these
<quote>extended</quote> REs) and obsolete REs (roughly those of
The <productname>Postgres</productname> formatting functions
provide a powerful set of tools for converting various data types
(date/time, integer, floating point, numeric) to formatted strings
- and for converting from formatted strings to specific datetypes.
+ and for converting from formatted strings to specific data types.
These functions all follow a common calling convention: The first
argument is the value to be formatted and the second argument is a
template that defines the output format.
<para>
A double quote (<quote><literal>"</literal></quote>) between
quotation marks is skipped and is not parsed. If you want to
- have a double quote in the output you must preceed it with a
+ have a double quote in the output you must precede it with a
double backslash, for example <literal>'\\"YYYY
Month\\"'</literal>. <!-- " font-lock sanity :-) -->
</para>
but to_char(-12, 'MI9999') produces <literal>'- 12'</literal>.
The Oracle implementation does not allow the use of
<literal>MI</literal> ahead of <literal>9</literal>, but rather
- requires that <literal>9</literal> preceed
+ requires that <literal>9</literal> precede
<literal>MI</literal>.
</para>
</listitem>
<title>Date/Time Functions</title>
<para>
- The date/time functions provide a powerful set of tools
- for manipulating various date/time types.
+ <xref linkend="functions-datetime-table"> shows the available
+ functions for date/time value processing. The basic arithmetic
+ operators (<literal>+</literal>, <literal>*</literal>, etc.) are
+ also available. For formatting functions, refer to <xref
+ linkend="functions-formatting">. You should be familiar with the
+ background information on date/time data types (see <xref
+ linkend="datatype-datetime">).
</para>
- <para>
- <table tocentry="1">
+ <table id="functions-datetime-table">
<title>Date/Time Functions</title>
<tgroup cols="4">
<thead>
<row>
- <entry>Function</entry>
- <entry>Returns</entry>
+ <entry>Name</entry>
+ <entry>Return Type</entry>
<entry>Description</entry>
<entry>Example</entry>
+ <entry>Result</entry>
</row>
</thead>
+
<tbody>
- <row>
- <entry>abstime(timestamp)</entry>
- <entry>abstime</entry>
- <entry>convert to abstime</entry>
- <entry>abstime(timestamp 'now')</entry>
- </row>
+<![IGNORE[
+<!--
+These two functions don't seem to do what it says here, or anything
+reasonable at all for that matter.
+-->
<row>
<entry>age(timestamp)</entry>
<entry>interval</entry>
- <entry>preserve months and years</entry>
+ <entry>Subtract argument from current date, preserve years and months and days</entry>
<entry>age(timestamp '1957-06-13')</entry>
+ <entry>43 years 8 mons 3 days</entry>
</row>
+
<row>
- <entry>age(timestamp,timestamp)</entry>
+ <entry>age(timestamp, timestamp)</entry>
<entry>interval</entry>
<entry>preserve months and years</entry>
<entry>age('now', timestamp '1957-06-13')</entry>
+ <entry></entry>
</row>
+]]>
+
<row>
- <entry>date_part(text,timestamp)</entry>
+ <entry>current_date</entry>
+ <entry>date</entry>
+ <entry>
+ returns current date; see also <link
+ linkend="functions-datetime-current">below</link>
+ </entry>
+ <entry></entry>
+ <entry></entry>
+ </row>
+
+ <row>
+ <entry>current_time</entry>
+ <entry>time</entry>
+ <entry>
+ returns current time (of day); see also <link
+ linkend="functions-datetime-current">below</link>
+ </entry>
+ <entry></entry>
+ <entry></entry>
+ </row>
+
+ <row>
+ <entry>current_timestamp</entry>
+ <entry>timestamp</entry>
+ <entry>
+ returns current date and time; see also <link
+ linkend="functions-datetime-current">below</link>
+ </entry>
+ <entry></entry>
+ <entry></entry>
+ </row>
+
+ <row>
+ <entry>date_part(text, timestamp)</entry>
<entry>double precision</entry>
- <entry>portion of date</entry>
- <entry>date_part('dow',timestamp 'now')</entry>
+ <entry>
+ extract subfield from date/time value (equivalent to
+ <function>extract</function>); see also <link
+ linkend="functions-datetime-datepart">below</link>
+ </entry>
+ <entry>date_part('hour', timestamp '2001-02-16 20:38:40')</entry>
+ <entry>20</entry>
</row>
+
<row>
- <entry>date_part(text,interval)</entry>
+ <entry>date_part(text, interval)</entry>
<entry>double precision</entry>
- <entry>portion of time</entry>
- <entry>date_part('hour',interval '4 hrs 3 mins')</entry>
+ <entry>
+ extract subfield from interval value (equivalent to
+ <function>extract</function>); see also <link
+ linkend="functions-datetime-datepart">below</link>
+ </entry>
+ <entry>date_part('month', interval '2 years 3 months')</entry>
+ <entry>3</entry>
</row>
+
<row>
- <entry>date_trunc(text,timestamp)</entry>
+ <entry>date_trunc(text, timestamp)</entry>
<entry>timestamp</entry>
- <entry>truncate date</entry>
- <entry>date_trunc('month',abstime 'now')</entry>
+ <entry>
+ truncate date to specified precision; see also <link
+ linkend="functions-datetime-trunc">below</link>
+ </entry>
+ <entry>date_trunc('hour', timestamp '2001-02-16 20:38:40')</entry>
+ <entry>2001-02-16 20:00:00+00</entry>
</row>
+
<row>
- <entry>interval(reltime)</entry>
- <entry>interval</entry>
- <entry>convert to interval</entry>
- <entry>interval(reltime '4 hours')</entry>
+ <entry>extract(identifier from timestamp)</entry>
+ <entry>double precision</entry>
+ <entry>
+ extract subfield from date/time value; see also <link
+ linkend="functions-datetime-extract">below</link>
+ </entry>
+ <entry>extract(hour from timestamp '2001-02-16 20:38:40')</entry>
+ <entry>20</entry>
</row>
+
+ <row>
+ <entry>extract(identifier from interval)</entry>
+ <entry>double precision</entry>
+ <entry>
+ extract subfield from interval value; see also <link
+ linkend="functions-datetime-extract">below</link>
+ </entry>
+ <entry>extract(month from interval '2 years 3 months')</entry>
+ <entry>3</entry>
+ </row>
+
<row>
<entry>isfinite(timestamp)</entry>
- <entry>bool</entry>
- <entry>a finite time?</entry>
- <entry>isfinite(timestamp 'now')</entry>
+ <entry>boolean</entry>
+ <entry>Returns true if the time stamp is finite (neither invalid nor infinity)</entry>
+ <entry>isfinite(timestamp '2001-02-16 21:28:30')</entry>
+ <entry>true</entry>
</row>
+
<row>
<entry>isfinite(interval)</entry>
- <entry>bool</entry>
- <entry>a finite time?</entry>
- <entry>isfinite(interval '4 hrs')</entry>
+ <entry>boolean</entry>
+ <entry>Returns true if the interval is finite in length</entry>
+ <entry>isfinite(interval '4 hours')</entry>
+ <entry>true</entry>
</row>
+
<row>
- <entry>reltime(interval)</entry>
- <entry>reltime</entry>
- <entry>convert to reltime</entry>
- <entry>reltime(interval '4 hrs')</entry>
+ <entry>now()</entry>
+ <entry>timestamp</entry>
+ <entry>
+ returns current date and time (equivalent to
+ <function>current_timestamp</function>); see also <link
+ linkend="functions-datetime-current">below</link>
+ </entry>
+ <entry></entry>
+ <entry></entry>
</row>
+
<row>
<entry>timestamp(date)</entry>
<entry>timestamp</entry>
- <entry>convert to timestamp</entry>
- <entry>timestamp(date 'today')</entry>
+ <entry>convert date to timestamp</entry>
+ <entry>timestamp(date '2000-12-25')</entry>
+ <entry>2000-12-25 00:00:00</entry>
</row>
+
<row>
- <entry>timestamp(date,time)</entry>
+ <entry>timestamp(date, time)</entry>
<entry>timestamp</entry>
- <entry>convert to timestamp</entry>
- <entry>timestamp(timestamp '1998-02-24',time '23:07');</entry>
- </row>
- <row>
- <entry>to_char(timestamp,text)</entry>
- <entry>text</entry>
- <entry>convert to string</entry>
- <entry>to_char(timestamp '1998-02-24','DD');</entry>
+ <entry>combine date and time into a timestamp</entry>
+ <entry>timestamp(date '1998-02-24',time '23:07')</entry>
+ <entry>1998-02-24 23:07:00</entry>
</row>
</tbody>
</tgroup>
</table>
+
+
+ <sect2 id="functions-datetime-extract">
+ <title><function>EXTRACT</function>, <function>date_part</function></title>
+
+<synopsis>
+EXTRACT (<replaceable>field</replaceable> FROM <replaceable>source</replaceable>)
+</synopsis>
+
+ <para>
+ The <function>extract</function> function retrieves sub-fields
+ from date/time values, such as year or hour.
+ <replaceable>source</replaceable> is a value expression that
+ evaluates to type <type>timestamp</type> or <type>interval</type>.
+ (Expressions of type <type>date</type> or <type>time</type> will
+ be cast to <type>timestamp</type> and can therefore be used as
+ well.) <replaceable>field</replaceable> is an identifier (not a
+ string!) that selects what field to extract from the source value.
+ The <function>extract</function> function returns values of type
+ <type>double precision</type>.
+ The following are valid values:
+
+ <!-- alphabetical -->
+ <variablelist>
+ <varlistentry>
+ <term>century</term>
+ <listitem>
+ <para>
+ The year field divided by 100
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
+</screen>
+ </informalexample>
+
+ <para>
+ Note that this is not really the century that the date is in.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>day</term>
+ <listitem>
+ <para>
+ The day (of the month) field (1 - 31)
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>decade</term>
+ <listitem>
+ <para>
+ The year field divided by 10
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(DECADE FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>200</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>dow</term>
+ <listitem>
+ <para>
+ The day of the week (0 - 6; Sunday is 0) (for
+ <type>timestamp</type> values only)
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>5</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>doy</term>
+ <listitem>
+ <para>
+ The day of the year (1 - 365/366) (for <type>timestamp</type> values only)
+ </para>
+ <informalexample>
+<screen>
+SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>47</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>epoch</term>
+ <listitem>
+ <para>
+ For <type>date</type> and <type>timestamp</type> values, the
+ number of seconds since 1970-01-01 00:00:00 (Result may be
+ negative.); for <type>interval</type> values, the total number
+ of seconds in the interval
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(EPOCH FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>982352320</computeroutput>
+
+SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
+<lineannotation>Result: </lineannotation><computeroutput>442800</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>hour</term>
+ <listitem>
+ <para>
+ The hour field (0 - 23)
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>microseconds</term>
+ <listitem>
+ <para>
+ The seconds field, including fractional parts, multiplied by 1
+ 000 000. Note that this includes full seconds.
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(MICROSECONDS FROM TIME '17:12:28.5');
+<lineannotation>Result: </lineannotation><computeroutput>28500000</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>millennium</term>
+ <listitem>
+ <para>
+ The year field divided by 1000
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>2</computeroutput>
+</screen>
+ </informalexample>
+
+ <para>
+ Note that this is not really the millennium that the date is in.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>milliseconds</term>
+ <listitem>
+ <para>
+ The seconds field, including fractional parts, multiplied by
+ 1000. Note that this includes full seconds.
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(MILLISECONDS FROM TIME '17:12:28.5');
+<lineannotation>Result: </lineannotation><computeroutput>28500</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>minute</term>
+ <listitem>
+ <para>
+ The minutes field (0 - 59)
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(MINUTE FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>38</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>month</term>
+ <listitem>
+ <para>
+ For <type>timestamp</type> values, the number of the month
+ within the year (1 - 12) ; for <type>interval</type> values
+ the number of months, modulo 12 (0 - 11)
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(MONTH FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>2</computeroutput>
+
+SELECT EXTRACT(MONTH FROM INTERVAL '2 years 3 months');
+<lineannotation>Result: </lineannotation><computeroutput>3</computeroutput>
+
+SELECT EXTRACT(MONTH FROM INTERVAL '2 years 13 months');
+<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>quarter</term>
+ <listitem>
+ <para>
+ The quarter of the year (1 - 4) that the day is in (for
+ <type>timestamp</type> values only)
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>second</term>
+ <listitem>
+ <para>
+ The seconds field, including fractional parts (0 -
+ 59<footnote><simpara>60 if leap seconds are
+ implemented by the operating system</simpara></footnote>)
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>40</computeroutput>
+
+SELECT EXTRACT(SECOND FROM TIME '17:12:28.5');
+<lineannotation>Result: </lineannotation><computeroutput>28.5</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>week</term>
+ <listitem>
+ <para>
+ From a <type>timestamp</type> value, calculate the number of
+ the week of the year that the day is in. By definition
+ (<acronym>ISO</acronym> 8601), the first week of a year
+ contains January 4 of that year. (The <acronym>ISO</acronym>
+ week starts on Monday.) In other words, the first Thursday of
+ a year is in week 1 of that year.
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>7</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>year</term>
+ <listitem>
+ <para>
+ The year field
+ </para>
+
+ <informalexample>
+<screen>
+SELECT EXTRACT(YEAR FROM TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>2001</computeroutput>
+</screen>
+ </informalexample>
+ </listitem>
+ </varlistentry>
+
+<!--
+tz
+tz_hour
+tz_minute
+-->
+
+ </variablelist>
+
</para>
<para>
- For the
- <function>date_part</function> and <function>date_trunc</function>
- functions, arguments can be
- `<literal>year</literal>', `<literal>month</literal>',
- `<literal>day</literal>', `<literal>hour</literal>',
- `<literal>minute</literal>', and `<literal>second</literal>',
- as well as the more specialized quantities
- `<literal>decade</literal>', `<literal>century</literal>',
- `<literal>millennium</literal>', `<literal>millisecond</literal>',
- and `<literal>microsecond</literal>'.
- <function>date_part</function> allows `<literal>dow</literal>'
- to return day of week, '<literal>week</literal>' to return the
- ISO-defined week of year, and `<literal>epoch</literal>' to return
- seconds since 1970 (for <type>timestamp</type>)
- or '<literal>epoch</literal>' to return total elapsed seconds
- (for <type>interval</type>).
+ The <function>extract</function> function is primarily intended
+ for computational processing. For formatting date/time values for
+ display, see <xref linkend="functions-formatting">.
</para>
- </sect1>
+
+ <anchor id="functions-datetime-datepart">
+ <para>
+ The <function>date_part</function> function is the traditional
+ <productname>Postgres</productname> equivalent to the
+ <acronym>SQL</acronym>-function <function>extract</function>:
+<synopsis>
+date_part('<replaceable>field</replaceable>', <replaceable>source</replaceable>)
+</synopsis>
+ Note that here the <replaceable>field</replaceable> value needs to
+ be a string. The valid field values for
+ <function>date_part</function> are the same as for
+ <function>extract</function>.
+ </para>
+
+ <informalexample>
+<screen>
+SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>
+
+SELECT date_part('hour', INTERVAL '4 hours 3 minutes')
+<lineannotation>Result: </lineannotation><computeroutput>4</computeroutput>
+</screen>
+ </informalexample>
+
+ </sect2>
+
+ <sect2 id="functions-datetime-trunc">
+ <title><function>date_trunc</function></title>
+
+ <para>
+ The function <function>date_trunc</function> is conceptually
+ similar to the <function>trunc</function> function for numbers.
+ </para>
+
+ <para>
+<synopsis>
+date_trunc('<replaceable>field</replaceable>', <replaceable>source</replaceable>)
+</synopsis>
+ <replaceable>source</replaceable> is a value expression of type
+ <type>timestamp</type> (values of type <type>date</type> and
+ <type>time</type> are cast automatically).
+ <replaceable>field</replaceable> selects to which precision to
+ truncate the time stamp value. The return value is of type
+ <type>timestamp</type> with all fields that are less than the
+ selected one set to zero (or one, for day and month).
+ </para>
+
+ <para>
+ Valid values for <replaceable>field</replaceable> are:
+ <simplelist>
+ <member>microseconds</member>
+ <member>milliseconds</member>
+ <member>second</member>
+ <member>minute</member>
+ <member>hour</member>
+ <member>day</member>
+ <member>month</member>
+ <member>year</member>
+ <member>decade</member>
+ <member>century</member>
+ <member>millennium</member>
+ </simplelist>
+ </para>
+
+ <informalexample>
+ <para>
+<screen>
+SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 20:00:00+00</computeroutput>
+
+SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
+<lineannotation>Result: </lineannotation><computeroutput>2001-01-01 00:00:00+00</computeroutput>
+</screen>
+ </para>
+ </informalexample>
+ </sect2>
+
+ <sect2 id="functions-datetime-current">
+ <title>Current Date/Time</title>
+
+ <para>
+ The following functions are available to select the current date and/or time:
+<synopsis>
+CURRENT_TIME
+CURRENT_DATE
+CURRENT_TIMESTAMP
+</synopsis>
+ Note that because of the requirements of the
+ <acronym>SQL</acronym> standard, these functions must not be
+ called with trailing parentheses.
+ </para>
+
+ <informalexample>
+<screen>
+SELECT CURRENT_TIME;
+<computeroutput>19:07:13</computeroutput>
+
+SELECT CURRENT_DATE;
+<computeroutput>2001-02-17</computeroutput>
+
+SELECT CURRENT_TIMESTAMP;
+<computeroutput>2001-02-17 19:07:32+00</computeroutput>
+</screen>
+ </informalexample>
+
+ <para>
+ The function <function>now()</function> is the traditional
+ <productname>Postgres</productname> equivalent to
+ <function>CURRENT_TIMESTAMP</function>.
+ <productname>Postgres</productname> furthermore has special
+ date/time <quote>constants</quote> that can be used to specify the
+ current time. The following three all return the same result:
+<programlisting>
+SELECT CURRENT_TIMESTAMP;
+SELECT now();
+SELECT TIMESTAMP 'now';
+</programlisting>
+ <note>
+ <para>
+ You do not want to use the third form when specifying a DEFAULT
+ value when creating a table. The system will immediately
+ evaluate the constant, thus when the default value is needed,
+ the time of the table creation would be used! The first two
+ forms will not be evaluated until the default value is used,
+ because they are function calls.
+ </para>
+ </note>
+ </para>
+ </sect2>
+ </sect1>
<sect1 id="functions-geometry">
</row>
<row>
<entry>isclosed(path)</entry>
- <entry>bool</entry>
+ <entry>boolean</entry>
<entry>a closed path?</entry>
<entry>isclosed(path '((0,0),(1,1),(2,0))')</entry>
</row>
<row>
<entry>isopen(path)</entry>
- <entry>bool</entry>
+ <entry>boolean</entry>
<entry>an open path?</entry>
<entry>isopen(path '[(0,0),(1,1),(2,0)]')</entry>
</row>
<title>Conditional Expressions</title>
<para>
- This section descibes the SQL-compliant conditional expressions
+ This section describes the <acronym>SQL</acronym>-compliant conditional expressions
available in <productname>Postgres</productname>.
</para>
<para>
The <acronym>SQL</acronym> <token>CASE</token> expression is a
generic conditional expression, similar to if/else statements in
- other languages. <token>CASE</token> clauses can be used whereever
+ other languages. <token>CASE</token> clauses can be used wherever
an expression is valid. <replaceable>condition</replaceable> is an
- expression that returns a boolean result. If the result is true
+ expression that returns a <type>boolean</type> result. If the result is true
then the value of the <token>CASE</token> expression is
<replaceable>result</replaceable>. If the result is false any
subsequent <token>WHEN</token> clauses are searched in the same
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