SQL语句行列转换

Posted 2020-06-17

创建表格

CREATE TABLE T2 
(STU_NAME CHAR(5),COURSE CHAR(5),MARK INT NOT NULL)
INSERT INTO T2 VALUES (‘城南‘,‘马哲‘,70)
INSERT INTO T2 VALUES (‘城南‘,‘数学‘,65)
INSERT INTO T2 VALUES (‘城南‘,‘英语‘,58)
INSERT INTO T2 VALUES (‘城南‘,‘语文‘,79)

INSERT INTO T2 VALUES (‘李四‘,‘马哲‘,61)
INSERT INTO T2 VALUES (‘李四‘,‘数学‘,80)
INSERT INTO T2 VALUES (‘李四‘,‘英语‘,77)
INSERT INTO T2 VALUES (‘李四‘,‘语文‘,80)

INSERT INTO T2 VALUES (‘王朝‘,‘马哲‘,52)
INSERT INTO T2 VALUES (‘王朝‘,‘数学‘,55)
INSERT INTO T2 VALUES (‘王朝‘,‘英语‘,59)
INSERT INTO T2 VALUES (‘王朝‘,‘语文‘,90)

INSERT INTO T2 VALUES (‘张三‘,‘马哲‘,66)
INSERT INTO T2 VALUES (‘张三‘,‘数学‘,88)
INSERT INTO T2 VALUES (‘张三‘,‘英语‘,61)
INSERT INTO T2 VALUES (‘张三‘,‘语文‘,70)

生成如下图表--

要求换列生成如下表格

 

方法一

select stu_name,
max(case course when ‘马哲‘ then mark else 0 end) as ‘马哲‘,
max(case course when ‘数学‘ then mark else 0 end) as ‘数学‘,
max(case course when ‘英语‘ then mark else 0 end) as ‘英语‘,
max(case course when ‘语文‘ then mark else 0 end) as ‘语文‘
from t2
group by stu_name
order by stu_name

 

 

方法二

SELECT 
A.STU_NAME AS ‘姓名‘,
A.MARK AS ‘马哲‘,
B.MARK AS ‘数学‘,
C.MARK AS ‘英语‘,
D.MARK AS ‘语文‘ 
FROM T2 AS A --把分数那列别名

INNER JOIN T2 AS B ON A.STU_NAME=B.STU_NAME 
INNER JOIN T2 AS C ON B.STU_NAME=C.STU_NAME 
INNER JOIN T2 AS D ON C.STU_NAME=D.STU_NAME
WHERE
A.COURSE=‘马哲‘ AND 
B.COURSE=‘数学‘ AND
C.COURSE=‘英语‘ AND 
D.COURSE=‘语文‘

方法三

--group by, avg/max, pivot。这里用max和avg，结果都一样，有什么区别吗？有点不明白
--参考网上的资料，用法如下
/*
pivot(
　　聚合函数(要转成列值的列名)
　　for 要转换的列
　　in（目标列名）
　　)
*/
select stu_name as ‘姓名‘,
avg(语文) as ‘马哲‘,
avg(数学) as ‘数学‘,
avg(英语) as ‘英语‘,
avg(语文) as ‘语文‘
from T2
pivot(
    avg(MARK) for course 
    in (马哲,数学,英语,语文)
    )as NewScores
group by stu_name
order by stu_name asc

--优化为

SELECT STU_NAME AS ‘姓名‘,
[马哲],[数学],[英语],[语文]
FROM T2
PIVOT(
MAX(MARK) FOR COURSE IN ([马哲],[数学],[英语],[语文])--以值变列,平均每个课程下的分数变成列
) AS NEW--一定要别名,否则报错

没有根据学生分类也会自动成功？？？