CONCAT() 函数

可以连接一或者多个字符串,不过其中如果有 Null 就直接返回 Null

mysql> select concat(11,22,33,44);
+---------------------+
| concat(11,22,33,44) |
+---------------------+
| 11223344            |
+---------------------+
1 row in set (0.00 sec)

mysql> select concat(11,22,null);
+--------------------+
| concat(11,22,null) |
+--------------------+
| NULL               |
+--------------------+
1 row in set (0.00 sec)

CONCAT_WS() 函数

以第一个占位符为分割,连接多个字符串,忽略 Null

mysql> select concat_ws('',11,22,null);
+--------------------------+
| concat_ws('',11,22,null) |
+--------------------------+
| 1122                     |
+--------------------------+
1 row in set (0.00 sec)

mysql> select concat_ws('-',11,22,null,33);
+------------------------------+
| concat_ws('-',11,22,null,33) |
+------------------------------+
| 11-22-33                     |
+------------------------------+
1 row in set (0.00 sec)

group_concat()函数

完整的语法如下：
group_concat([DISTINCT] 要连接的字段 [Order BY ASC/DESC 排序字段] [Separator '分隔符'])

基本查询

mysql> select * from aa;
+------+------+
|id | name   |
+------+------+
|1  | 10     |
|1  | 20     |
|1  | 20     |
|2  | 20     |
|3  | 200    |
|3  | 500    |
+------+------+
6 rows in set (0.00 sec)

以id分组，把name字段的值打印在一行，逗号分隔(默认)

mysql> select id,group_concat(name) from aa group by id;
+------+--------------------+
| id| group_concat(name) |
+------+--------------------+
|1 | 10,20,20|
|2 | 20 |
|3 | 200,500|
+------+--------------------+
3 rows in set (0.00 sec)

以id分组，把name字段的值打印在一行，分号分隔

mysql> select id,group_concat(name separator ';') from aa group by id;
+------+----------------------------------+
| id| group_concat(name separator ';') |
+------+----------------------------------+
|1 | 10;20;20 |
|2 | 20|
|3 | 200;500   |
+------+----------------------------------+
3 rows in set (0.00 sec)

以id分组，把去冗余的name字段的值打印在一行，

逗号分隔

mysql> select id,group_concat(distinct name) from aa group by id;
+------+-----------------------------+
| id| group_concat(distinct name) |
+------+-----------------------------+
|1 | 10,20|
|2 | 20   |
|3 | 200,500 |
+------+-----------------------------+
3 rows in set (0.00 sec)

以id分组，把name字段的值打印在一行，逗号分隔，以name排倒序

mysql> select id,group_concat(name order by name desc) from aa group by id;
+------+---------------------------------------+
| id| group_concat(name order by name desc) |
+------+---------------------------------------+
|1 | 20,20,10   |
|2 | 20|
|3 | 500,200|
+------+---------------------------------------+
3 rows in set (0.00 sec)

repeat()函数

用来复制字符串,如下'ab'表示要复制的字符串，2表示复制的份数

mysql> select repeat('ab',2);
+----------------+
| repeat('ab',2) |
+----------------+
| abab           |
+----------------+

1 row in set (0.00 sec)

又如

mysql> select repeat('a',2);

+---------------+
| repeat('a',2) |
+---------------+
| aa            |
+---------------+
1 row in set (0.00 sec)

参照文章: Mysql中concat函数（连接字符串）