PostgreSQL provides a large number of
functions and operators for the built-in data types. Users can also
define their own functions and operators, as described in the
PostgreSQL 7.3 Programmer's Guide. The
psql commands \df and
\do can be used to show the list of all actually
available functions and operators, respectively.
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. Some of this extended functionality is present in other
SQL implementations, and in many cases this
functionality is compatible and consistent between various products.
The usual logical operators are available:
SQL uses a three-valued Boolean logic where the null value represents
"unknown". Observe the following truth tables:
