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

The SET statement in stored routines is an extended version of the
general SET statement. Referenced variables may be ones declared inside
a routine, or global system variables.

The SET statement in stored routines 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 and
global and session server 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/5.0/en/set-statement.html

