Variables
mysql> SET @t1=1, @t2=2, @t3:=4;
mysql> SELECT @t1, @t2, @t3, @t4 := @t1+@t2+@t3;
For SET, either = or := can be used as the assignment operator. You
can also assign a value to a user variable in statements other than SET.
In this case, the assignment operator must be := and not = because the
latter is treated as the comparison operator = in non-SET statements