Mysql之如何实现行列转换
作者:蓝天⊙白云
这篇文章主要介绍了Mysql之如何实现行列转换问题,具有很好的参考价值,希望对大家有所帮助。如有错误或未考虑完全的地方,望不吝赐教
mysql数据库如何实现行列转换
1. 行转列
方案一
select name, sum(case when course='java' then grade end) as java, sum(case when course='C++' then grade end) as C++, sum(case when course='C#' then grade end) as C# from test group by name
方案二
select distinct c.`name` AS name, (select grade from test where name = c.`name` and course = 'java' )as java, (select grade from test where name = c.`name` and course = 'C++' )as C++, (select grade from test where name = c.`name` and course = 'C#' )as C# from test c
结合项目用到的sql:
select MAIN_STATION_CODE_ as stationCode, case when SUBSTR(PLAN_START_DATE_, 1, 10)=curdate() then STATION_ else "" end as firstDay, case when SUBSTR(PLAN_START_DATE_, 1, 10)=DATE_SUB(curdate(),INTERVAL -1 DAY) then STATION_ else "" end as secondDay, case when SUBSTR(PLAN_START_DATE_, 1, 10)=DATE_SUB(curdate(),INTERVAL -2 DAY) then STATION_ else "" end as thirdDay, case when SUBSTR(PLAN_START_DATE_, 1, 10)=DATE_SUB(curdate(),INTERVAL -3 DAY) then STATION_ else "" end as fourthDay from ps_overhaul_plan_row group by MAIN_STATION_CODE_
方案二拓展
上面我们是采用逐个判断并拼接的方式来进行,那如果极端情况下,假如有成百上千个值需要判断怎么办?
这种情况下,能够立即想到的是采用函数来拼接出对应的语句块,再合并在一起执行。
假如我们需要拼接下面一句:
(select GRADE_ FROM grade where name_ = c.name_ and course_ = ‘Math') as Math,
使用concat函数(例子):
select c.name_,concat('(SELECT grade_ from grade where name_=c.name_ ', 'and course_ =''', c.course_, '''', ') as ',c.course_, ')') from grade c
然后再次使用group_concat函数将多行转为一行:
SELECT GROUP_CONCAT(distinct concat('(SELECT grade_ from grade where name_=c.name_ ', 'and course_ =''', c.course_, '''', ') as ',c.course_, ')')) from grade c
最后再使用存储过程完成动态sql执行。
方案三:带汇总
select ifnull(uid,'Total') uid, uname, sum(if(`course`='java',grade,0)) 'java', sum(if(`course`='C++',grade,0)) 'C++', sum(if(`course`='C#',grade,0)) 'C#', sum(score) 'total' from course group by uid with ROLLUP
方案四:使用group_concat函数
SELECT id, name, group_concat(CASE WHEN subject = 'Math' THEN score END SEPARATOR '') '数学', group_concat(CASE WHEN subject = 'English' THEN score END SEPARATOR '') '英语' FROM test1 GROUP BY name;
2. 列转行
原表:
实现SQL:
SELECT d.name_,'Math' AS subject,d.Math_ AS score FROM grade_column d UNION ALL SELECT d.name_, 'English' as SUBJECT,d.English_ as score from grade_column d
列转行效果如下:
说明:SELECT “hello” as subject from dual;作用是新添加列subject,并且列值为hello;
总结
行转列原理:
CASE WHEN或IF,这两种都是判断条件,满足条件的时候我们把它当做新的一列。
列转行原理:
UNION或UNION ALL,这两个都是把结果集合并起来,每次查询学生名称(基本列)和学科的其中一列的值,再把它们组合起来,这样结果集就只有学生名称和科目成绩两列了,这里多加了一列科目。
以上为个人经验,希望能给大家一个参考,也希望大家多多支持脚本之家。