## Chapter 9. Functions and Operators

Postgres Pro 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 Part V. The psql commands `\df` and `\do` can be used to list all available functions and operators, respectively.

If you are concerned about portability then 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 database management systems, and in many cases this functionality is compatible and consistent between the various implementations. This chapter is also not exhaustive; additional functions appear in relevant sections of the manual.

## 9.1. Logical Operators

The usual logical operators are available:

`AND`
`OR`
`NOT`

SQL uses a three-valued logic system with true, false, and `null`, which 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

The operators `AND` and `OR` are commutative, that is, you can switch the left and right operand without affecting the result. But see Section 4.2.14 for more information about the order of evaluation of subexpressions.