如果在字符串位置全部为NULL,则MySQL MAKE_SET()函数将不返回任何内容。以下示例将演示它-
mysql> Select MAKE_SET(2, NULL,NULL,NULL); +-----------------------------+ | MAKE_SET(2, NULL,NULL,NULL) | +-----------------------------+ | | +-----------------------------+ 1 row in set (0.00 sec)