PostgreSQL 7.1.3 Documentation
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# Chapter 4. Functions and Operators

Postgres 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 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. However, many other RDBMS packages provide a lot of the same or similar functions, and some of the ones provided in Postgres have in fact been inspired by other implementations.

# 4.1. Logical Operators

The usual logical operators are available:

AND
OR
NOT

SQL uses a three-valued Boolean logic where NULL represents "unknown". Observe the following truth tables:

`a` | `b` | `a` AND `b` | `a` OR `b`
--- | --- | --- | ---
TRUE | TRUE | TRUE | TRUE
TRUE | FALSE | FALSE | TRUE
TRUE | NULL | NULL | TRUE
FALSE | FALSE | FALSE | FALSE
FALSE | NULL | FALSE | NULL
NULL | NULL | NULL | NULL

`a` | NOT `a`
--- | ---
TRUE | FALSE
FALSE | TRUE
NULL | NULL

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