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mysql> select sum(if(type=1,1,-1)*num)
-> from t75;
+--------------------------+
| sum(if(type=1,1,-1)*num) |
+--------------------------+
| 0 |
+--------------------------+
1 row in set (0.00 sec)
mysql>
mysql> select * from t75;
+------+------+------+
| id | num | type |
+------+------+------+
| 1 | 30 | 1 |
| 2 | 40 | 2 |
| 3 | 10 | 2 |
| 4 | 20 | 1 |
+------+------+------+
4 rows in set (0.00 sec)
mysql> select sum((3-type*2)*num) as tsum
-> from t75;
+------+
| tsum |
+------+
| 0 |
+------+
1 row in set (0.00 sec)
mysql>