A procedure is a database object similar to a function. The key differences are:
Procedures are defined with
the CREATE
PROCEDURE 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 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
and DROP ROUTINE that can operate on functions and
procedures without having to know which kind it is. Note, however, that
there is no CREATE ROUTINE command.