MySQL查询技巧

1.行转列

SELECT user_name,
    MAX(CASE course WHEN "math" THEN score ELSE 0 END) AS "math", 
    MAX(CASE course WHEN "English" THEN score ELSE 0 END) AS "English", 
    MAX(CASE course WHEN "Chinese" THEN score ELSE 0 END) AS "Chinese"
    FROM tb GROUP BY user_name

2.列转行

select user_name, 'math' course, math_score as score from tb
    union select user_name, 'English' course English_score as score from tb
    union select user_name, 'Chinese' course Chinese_score as score from tb
    order by user_name, course;

3.在子查询中实现多列过滤

• 单列：

select * from person where name in (select name from job)

• 多列：

select * from person where(name, sex) in (select name, sex from job)

4.同一属性的多值过滤

select a.no, a.name, b.subject, b.score, c.subject, c.score from student a 
    join stscore b on a.no = b.stno 
    join stscore c on b.stno = c.stno
    and b.subject='math' and b.score>85 and c.subject ='English' and c.score>85;

• 使用关联进行查询

select a.name, b.subject, b.score, c.subject, c.score, d.subject, d.score from student a
  left join stscore b on a.no = b.stno and b.subject = 'math' and b.score > 85 
  left join stscore c on a.no = c.stno and c.subject = 'English' and c.score > 85
  left join stscore d on a.no = d.stno and d.subject = 'Chinese' and d.score > 85
  where(case when b.subject is not null then 1 else 0 end) +
       (case when c.subject is not null then 1 else 0 end) +
       (case when d.subject is not null then 1 else 0 end) >= 2

• 使用Group by实现查询

select a.name from student a join stscore b on a.id = b.stno where
    b.subject in ('math', 'English', 'chinese') and b.score > 0 
    group by a.name having count(*) >= 2