# 处理 `NULL`

__*NULL 的行为*__

+ 传递 NULL 到 CONCAT 函数, 导致它返回 NULL 
+ CONCAT_WS() 忽略 NULL 参数
+ 例如:
  + `select concat('Welcome ', 'to ', NULL), concat_ws(' ','Welcome','to', NULL);` 
```
tidb> SELECT concat('Welcome ', 'to ', NULL), concat_ws(' ','Welcome','to' ,NULL);
+---------------------------------+------------------------------------+
| concat('Welcome ', 'to ', NULL) | concat_ws(' ','Welcome','to',NULL) |
+---------------------------------+------------------------------------+
| ~!@NULL@!~                            | Welcome to                         |
+---------------------------------+------------------------------------+
1 row in set (0.00 sec)
```
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