concat和concat_ws()区别及group_concat() ,repeat()字符串函数
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1、
concat
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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 ) |