concat和concat_ws()区别及group_concat() ,repeat()字符串函数_MySQL

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           |
        +--------------+
       1 row in set (0.00 sec)

       mysql> select concat('11','22','33');
        +------------------------+
        | concat('11','22','33') |
        +------------------------+
        | 112233                 |
        +------------------------+

       1 row in set (0.00 sec)

       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                   |
        +------------------------+
       1 row in set (0.00 sec)

       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                      |
        +-------------------------------+
       1 row in set (0.00 sec)

   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                         |
        +-------------------------------+
       1 row in set (0.00 sec)

3、group_concat()可用来行转列, Oracle没有这样的函数 （mysql4.1及以上 ）

    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)

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