MYSQL函数用法—

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

    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)

select org_id, group_concat(distinct(job_name ) order by job_name desc separator'|')as  name from v_job_search v  group by org_id-->对表v_job_search用“|”分割后还可排序。

例如：“Web Developer |Project Manager |Flash Developer |Account Service Manager”

select concat_ws('|',job_name,company_name) as name from v_job_search;-->对表v_job_search 中两个字段进行连接用‘|’分割；例如：“Electrical Engineer|Mosher & Associates”

如果对表中的字段统计，如果要把统计结果和某个字段连接要把num 转化字符才可以连接。

SELECT  IF (t.num > 1, concat(t.org_name, ' - ', CAST(t.num AS CHAR),' Jobs'),concat(t.org_name, ' - ', CAST(t.num AS CHAR),' Job')) AS dispayName