在链接字符串时，如果我将添加NULL值，那么CONCAT_WS()函数的输出将是什么？
实际上，当且仅当它的第一个参数（即分隔符）为NULL时，CONCAT_WS()函数才返回NULL。一个例子如下-
mysql> Select CONCAT_ws(NULL,'Tutorial','Point','.com'); +-------------------------------------------+ | CONCAT_ws(NULL,'Tutorial','Point','.com') | +-------------------------------------------+ | NULL | +-------------------------------------------+ 1 row in set (0.00 sec)
否则，如果我们在链接字符串时将NULL放在CONCAT_WS()函数的任何其他位置，则MySQLCONCAT_WS()函数将忽略NULL。以下示例将展示它-
mysql> Select CONCAT_ws('s','Tutorial','Point','.com',NULL);
+-----------------------------------------------+
| CONCAT_ws('s','Tutorial','Point','.com',NULL) |
+-----------------------------------------------+
| nhooos.com |
+-----------------------------------------------+
1 row in set (0.00 sec)
mysql> Select CONCAT_ws('s','Tutorial',NULL,'Point','.com');
+-----------------------------------------------+
| CONCAT_ws('s','Tutorial',NULL,'Point','.com') |
+-----------------------------------------------+
| nhooos.com |
+-----------------------------------------------+
1 row in set (0.00 sec)