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]). Each var_name may refer to a
local variable declared inside a stored program, a system variable, or
a user-defined variable.

The SET statement in stored programs is implemented as part of the
pre-existing SET syntax. This enables an extended syntax of SET a=x,
b=y, ... where different variable types (locally declared variables,
global and session system variables, user-defined variables) can be
mixed. This also enables 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/5.5/en/set-statement.html

