如果字符串处全为 NULL，MySQL MAKE_SET() 函数将不会返回任何内容。以下示例将演示它 -

示例

mysql> Select MAKE_SET(2, NULL,NULL,NULL);
+-----------------------------+
| MAKE_SET(2, NULL,NULL,NULL) |
+-----------------------------+
|                             |
+-----------------------------+
1 row in set (0.00 sec)