Syntax:
SET var_name = expr [, var_name = expr] ...

The SET statement in stored programs is an extended version of the
general SET statement (see [HELP SET]). Referenced variables may be
ones declared inside a stored program, global system variables, or
user-defined variables.

The SET statement in stored programs is implemented as part of the
pre-existing SET syntax. This allows an extended syntax of SET a=x,
b=y, ... where different variable types (locally declared variables,
global and session server variables, user-defined variables) can be
mixed. This also allows combinations of local variables and some
options that make sense only for system variables; in that case, the
options are recognized but ignored.

URL: http://dev.mysql.com/doc/refman/5.1/en/set-statement.html

