set @a:=1, @b:=2, @c:=3;
set @max:=greatest(@a, @b, @c);
set @min:=least(@a, @b, @c);
select @max, @min;
для установки в SP,
delimiter $$
create procedure extremevalues ( in a1 int, in a2 int ,
in a3 int, in a4 int,
in a5 int, min_a int, max_a int)
begin
set max_a:=greatest(a1, a2, a2, a3, a4, a5);
set min_a:=least(a1, a2, a2, a3, a4, a5);
select max_a, min_a;
end$$