# 38.4. User-defined Procedures

A procedure is a database object similar to a function. The key differences are:

* Procedures are defined with the [`CREATE PROCEDURE`](https://www.postgresql.org/docs/current/sql-createprocedure.html) command, not `CREATE FUNCTION`.
* Procedures do not return a function value; hence `CREATE PROCEDURE` lacks a `RETURNS` clause. However, procedures can instead return data to their callers via output parameters.
* While a function is called as part of a query or DML command, a procedure is called in isolation using the [`CALL`](https://www.postgresql.org/docs/current/sql-call.html) command.
* A procedure can commit or roll back transactions during its execution (then automatically beginning a new transaction), so long as the invoking `CALL` command is not part of an explicit transaction block. A function cannot do that.
* Certain function attributes, such as strictness, don't apply to procedures. Those attributes control how the function is used in a query, which isn't relevant to procedures.

The explanations in the following sections about how to define user-defined functions apply to procedures as well, except for the points made above.

Collectively, functions and procedures are also known as *routines*. There are commands such as [`ALTER ROUTINE`](https://www.postgresql.org/docs/current/sql-alterroutine.html) and [`DROP ROUTINE`](https://www.postgresql.org/docs/current/sql-droproutine.html) that can operate on functions and procedures without having to know which kind it is. Note, however, that there is no `CREATE ROUTINE` command.