MySQL EXPORT_SET()函数如果任何一个参数为空，会返回什么？

如果在 MySQL EXPORT_SET() 函数中提供了任何参数，其中有一个为 NULL，该函数将返回 NULL。以下示例将说明它：

阅读更多：MySQL 教程

示例

mysql> Select EXPORT_SET(NULL, 'Y','N',',', 4);

+----------------------------------+
| EXPORT_SET(NULL, 'Y','N',',', 4) |
+----------------------------------+
| NULL                             |
+----------------------------------+

1 row in set (0.00 sec)

mysql> Select EXPORT_SET(5, NULL,'N',',', 4);

+--------------------------------+
| EXPORT_SET(5, NULL,'N',',', 4) |
+--------------------------------+
| NULL                           |
+--------------------------------+

1 row in set (0.00 sec)

mysql> Select EXPORT_SET(5, 'Y','N',',', NULL);

+----------------------------------+
| EXPORT_SET(5, 'Y','N',',', NULL) |
+----------------------------------+
| NULL                             |
+----------------------------------+

1 row in set (0.00 sec)

mysql> Select EXPORT_SET(5, 'Y','N',NULL, 4);

+--------------------------------+
| EXPORT_SET(5, 'Y','N',NULL, 4) |
+--------------------------------+
| NULL                           |
+--------------------------------+

1 row in set (0.00 sec)