concat和concat_ws()区别Oracle及MySQL的几个实用字符串函数 concat_ws

1、concat()函数

1.1 MySQL的concat函数可以连接一个或者多个字符串,如

mysql>
select concat('10');

+--------------+

| concat('10') |

+--------------+

| 10

+--------------+

mysql> select concat('11','22','33');

+------------------------+

| concat('11','22','33') |

+------------------------+

| 112233 |

+------------------------+

而Oracle的concat函数只能连接两个字符串

SQL> select concat('11','22') from dual;

1.2 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 |

+------------------------+

而Oracle的concat函数连接的时候，只要有一个字符串不是NULL,就不会返回NULL

SQL> select concat('11',NULL) from dual;

CONCAT

--

11

2、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 |

+-------------------------------+

和concat不同的是, concat_ws函数在执行的时候,不会因为NULL值而返回NULL

mysql> select concat_ws(',','11','22',NULL);

+-------------------------------+

| concat_ws(',','11','22',NULL) |

+-------------------------------+

| 11,22 |

+-------------------------------+

3、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 |

+---------------+