In this case, the assignment operator must be := and not = because = is treated as a comparison operator in non-SET statements:
mysql>mysql> SET @t1=0, @t2=1, @t3=2;Query OK, 0 rows affected (0.00 sec)mysql>mysql> SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;+----------------------+------+------+------+| @t1:=(@t2:=1)+@t3:=4 | @t1 | @t2 | @t3 |+----------------------+------+------+------+| 5 | 5 | 1 | 4 |+----------------------+------+------+------+1 row in set (0.00 sec)mysql>
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