mysql concat和group_concat

mysql concat(str1,str2...)连接两个字符串，（数字也是可以的，会转成字符串）

MySQL的concat函数在连接字符串的时候，只要其中一个是NULL,那么将返回NULL

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

concat_ws()函数, 表示concat with separator,即有分隔符的字符串连接
如连接后以逗号分隔
mysql> select concat_ws(',','11','22','33');
 
+-------------------------------+
| concat_ws(',','11','22','33') |
+-------------------------------+
| 11,22,33                      |
+-------------------------------+
1 row in set (0.00 sec)

现在有这样一个需求，

要查找用户名和密码为(user1,pwd1),(user2,pwd2),(user2,plwd3)

由于字段与字段之间有关联，我们就可以使用连接解决这个问题：

select * from user where concat(username,'-',pwd) in ('user1-pwd1','user2-pwd2')

 

group_concat()可用来行转列, Oracle没有这样的函数
 
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'])
如下例子
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)
3.1 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)
 
3.2 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)
 
3.3 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)
 
3.4 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)
 
4、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)