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<chapter id="functions">

<title>Functions and Operators</title>

This expression yields true when two time periods (defined by their

endpoints) overlap, false when they do not overlap. The endpoints

can be specified as pairs of dates, times, or time stamps; or as

a date, time, or time stamp followed by an interval.

a date, time, or time stamp followed by an interval. When a pair

of values is provided, either the start or the end can be written

first; <literal>OVERLAPS</> automatically takes the earlier value

of the pair as the start. Each time period is considered to

represent the half-open interval <replaceable>start</> <literal>&lt;=</>

<replaceable>time</> <literal>&lt;</> <replaceable>end</>, unless

<replaceable>start</> and <replaceable>end</> are equal in which case it

represents that single time instant. This means for instance that two

time periods with only an endpoint in common do not overlap.

</para>

<screen>

SELECT (DATE '2001-02-16', INTERVAL '100 days') OVERLAPS

(DATE '2001-10-30', DATE '2002-10-30');

<lineannotation>Result: </lineannotation><computeroutput>false</computeroutput>

SELECT (DATE '2001-10-29', DATE '2001-10-30') OVERLAPS

(DATE '2001-10-30', DATE '2001-10-31');

<lineannotation>Result: </lineannotation><computeroutput>false</computeroutput>

SELECT (DATE '2001-10-30', DATE '2001-10-30') OVERLAPS

(DATE '2001-10-30', DATE '2001-10-31');

<lineannotation>Result: </lineannotation><computeroutput>true</computeroutput>

</screen>

<para>