MySQL FIND_IN_SET()函数何时返回NULL作为输出？
如果任何参数（即搜索字符串或字符串列表）为NULL，则FIND_IN_SET()函数将返回NULL作为输出。当然，如果两个参数都为NULL，它将也返回NULL。
示例
mysql> Select FIND_IN_SET(NULL,'Ram is a good boy') AS Result;
+--------+
| Result |
+--------+
| NULL |
+--------+
1 row in set (0.00 sec)
mysql> SELECT FIND_IN_SET('RAM',NULL)AS RESULT;
+--------+
| RESULT |
+--------+
| NULL |
+--------+
1 row in set (0.00 sec)
mysql> SELECT FIND_IN_SET(NULL,NULL);
+------------------------+
| FIND_IN_SET(NULL,NULL) |
+------------------------+
| NULL |
+------------------------+
1 row in set (0.00 sec)