mysql 字符拼接

《TiDB v2.0 中文技术文档》

• https://www.bookstack.cn/read/pingcap-docs-cn/sql-string-functions.md

一、CONCAT

• 功能：无缝拼接列或者字符 ；
• 语法：CONCAT(string1,string2,...)
• 说明：此方法在拼接的时候如果有一个值为NULL，则返回NULL

二、CONCAT_WS 

• 功能：可以多个字符串用指定的字符串进行拼接 ；
• 语法：CONCAT_WS(separator,string1,string2,...)

 三、GROUP_CONCA

• 功能：行转列 合并显示(返回连接的字符串) ；
• 语法：GROUP_CONCAT([DISTINCT] 要连接的字段 [Order BY ASC/DESC 排序字段] [Separator '分隔符']）
• 说明：mysql group_concat( order by 可以正常排序) ,但在 TiDB 中存在排序异常 ； 

select id,group_concat( score order by score desc SEPARATOR ',') as unionStr,school
from (
select '1' as id,90 as score , 'a' as school union all 
select '1' as id,91 as score , 'a' as school union all 
select '1' as id,92 as score , 'a' as school union all 
select '2' as id,94 as score , 'a' as school union all 
select '2' as id,95 as score , 'a' as school union all 
select '2' as id,96 as score , 'a' as school union all 
select '3' as id,97 as score , 'a' as school union all 
select '3' as id,98 as score , 'a' as school union all 
select '3' as id,99 as score , 'a' as school 
)t  
group by 1,3 ;