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

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 )