0x01 concat()

注意：返回结果为连接参数产生的字符串，如果有任何一个参数为null，则返回值为null

用途：连接一个或多个字符串
语法：concat(str1,str2…)

# 不带NULL的
mysql> select concat('123','456','aaa','789');
+---------------------------------+
| concat('123','456','aaa','789') |
+---------------------------------+
| 123456aaa789                    |
+---------------------------------+

# 带NULL的
mysql> select concat('123','456',NULL,'aaa','789');
+--------------------------------------+
| concat('123','456',NULL,'aaa','789') |
+--------------------------------------+
| NULL                                 |
+--------------------------------------+
1 row in set

0x02 concat_ws()

注意：如果 concat_ws 的参数有NULL会无视掉

用途：使用固定连接符连接一个或多个字符串
语法：concat_ws(separator,str1,str2…)

# 不带NULL的
mysql> select concat_ws('-','aa','bb','cc');
+------------------------+
| concat_ws('-','aa','bb','cc') |
+------------------------+
| aa-bb-cc               |
+------------------------+
1 row in set

# 带NULL的
mysql> select concat_ws('-','aa','bb',NULL,'cc');
+-----------------------------+
| concat_ws('-','aa','bb',NULL,'cc') |
+-----------------------------+
| aa-bb-cc                    |
+-----------------------------+
1 row in set