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)
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)
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)