Inner join
#(inner) join 选择两个表里面的交集，下面查找所有student表里面所有学生的成绩信息，
#没有包含没有参加考试的李四，也没有列出student表不存在的007号学生
SELECT a.name,a.grade,b.subject,b.score FROM student a INNER JOIN scores b on a.uid= b.sid;
inner join.png
left join
#LEFT JOIN 已左表为基准，如果条件不成立，被join表的字段为空，放开where条件后结果同inner join，没有参加考试的学生成绩也出来了
SELECT a.name,a.grade,b.subject,b.score FROM student a
LEFT JOIN scores b on a.uid = b.sid -- where not ISNULL(b.`subject`);
left join.png
join也可以优化子查询。比如如果查询每个学生参加考试的姓名，班级和科目信息，在这个案例中如果一个学生只参加一个科目就可以用下面语句查询，但是每次从a查询一条数据都要从b查询对应科目效率地下
SELECT a.name,a.grade,(SELECT b.`subject` FROM scores b where a.uid = b.sid limit 1) FROM student a;
用join可以优化查询，而且join比上面的子查询适用范围还广，不一定要一对一。
SELECT a.name,a.grade ,b.subject FROM student a JOIN scores b ON a.uid = b.sid
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right join
#LEFT JOIN 已被join表为基准，如果条件不成立，左表的字段为空 ,下面一个未知考生的成绩也出来了
SELECT a.name,a.grade,b.subject,b.score FROM student a RIGHT JOIN scores b on a.uid = b.sid -- where not ISNULL(b.`subject`);
right join.png
full join
求并集
#mysql不支持full join，可以用union all left join和right join的结果
SELECT a.name,a.grade,b.subject,b.score FROM student a
RIGHT JOIN scores b on a.uid = b.sid
union
SELECT a.name,a.grade,b.subject,b.score FROM student a
LEFT JOIN scores b on a.uid = b.sid
优化聚合子查询
SELECT a.name,a.grade,b.subject,b.score FROM student a
LEFT JOIN scores b ON a.uid = b.sid
where b.score = (SELECT MAX(c.score) FROM scores c WHERE c.sid = b.sid)
上面的查询条件where里面有子查询，可以通过join方式优化上面的聚合查询
SELECT a.name,a.grade,b.`subject`,b.score FROM student a
JOIN scores b ON a.uid = b.sid
JOIN scores c ON b.sid = c.sid
GROUP BY a.name,b.`subject`,b.score
HAVING b.score = MAX(c.score)
最高成绩.png
分组选择
查询每个人考试成绩最高的前两科成绩
下面这个在子查询中适用limit提示不支持
SELECT a.name,a.grade,b.subject,b.score FROM student a LEFT JOIN scores b ON a.uid = b.sid
where b.score in (SELECT c.score FROM scores c WHERE c.sid = b.sid limit 2)
可以依次执行下面的语句，对每个人执行依次,但是意味着有多少人就要执行多少次，效率很低。
SELECT a.name,a.grade,b.subject,b.score FROM student a LEFT JOIN scores b ON a.uid = b.sid
where a.`name` = '张三' ORDER BY b.score limit 2
可以用下面一条语句查询出来
SELECT d.name,c.subject,score
FROM (SELECT sid,subject,score,
(SELECT COUNT(*) FROM scores b WHERE b.sid = a.sid and a.score <= b.score) as cnt
FROM scores a
GROUP BY sid,`subject`,score) c JOIN student d on c.sid = d.uid
where cnt <= 2
行转列
可以使用cross join
SELECT* FROM
(SELECT SUM(score) as '张三' FROM student a JOIN scores b ON a.uid = b.sid and a.`name` = '张三') a CROSS JOIN
(SELECT SUM(score) as '王五' FROM student a JOIN scores b ON a.uid = b.sid and a.`name` = '王五') b CROSS JOIN
(SELECT SUM(score) as '赵六' FROM student a JOIN scores b ON a.uid = b.sid and a.`name` = '赵六' ) c
用case 语句会更好实现
SELECT
SUM(CASE WHEN `name` = '张三' then score end) as 张三,
SUM(CASE WHEN `name` = '王五' then score end) as 王五,
SUM(CASE WHEN `name` = '赵六' then score end) as 赵六
FROM student JOIN scores ON student.uid = scores.sid;
image.png
列转行
下面的代码适用于把一个人有多个手机号，用逗号隔开。拆分成一个姓名对应一个手机号的情况。
序列化表.png
唯一序列号SQL
image.png
删除重复数据
假设某个学生参加考试成绩有重复的，去掉重复的，保留最好的成绩
image.png
SELECT a.* FROM scores a JOIN
(SELECT sid,`subject`,COUNT(*) cnt,MAX(score) max_score FROM scores GROUP BY sid,`subject`) b
ON a.`subject` = b.`subject` and a.sid = b.sid and b.cnt > 1 and a.score < b.max_score
用条件子查询也可以实现
SELECT* FROM scores WHERE (sid,`subject`) in
(select sid,`subject` FROM scores GROUP BY sid,`subject` HAVING COUNT(*) > 1)
and score not in (SELECT MAX(score) FROM scores GROUP BY sid,`subject` HAVING COUNT(*) > 1)
image.png
上面选择是正确的但是删除会报错：
you can't specify target table for update in from clause
所以可以将查询需要删除的id放在一个中间表就可以了。
DELETE FROM scores WHERE uid in (SELECT uid FROM (
SELECT a.uid FROM scores a JOIN
(SELECT sid,`subject`,COUNT(*) cnt,MAX(score) max_score FROM scores GROUP BY sid,`subject`) b
ON a.`subject` = b.`subject` and a.sid = b.sid and b.cnt > 1 and a.score < b.max_score
)c)
多属性查询
查询英语和数学成绩同时在80以上的人的信息
SELECT a.name,b.subject,b.score FROM student a left JOIN scores b on a.uid = b.sid
WHERE b.subject in ('英语','数学') and b.score > 80
这是错误的，因为有些人根本没有参加英语考试，in的条件是只要在集合中有一个符合就可以，所以上述查出的是英语或数学在80以上的人的信息
可以用 join 求交集来实现。
SELECT a.name,b.subject,b.score,c.subject,c.score FROM student a
JOIN scores b on a.uid = b.sid and b.subject = '数学' and b.score > 80
JOIN scores c ON b.sid = c.sid and c.subject = '英语' and c.score > 80
求参加语数英里面至少两门以上的考生的信息
SELECT a.name ,b.subject as '数学',c.subject as '语文',d.subject as '英语' FROM student a
LEFT JOIN scores b on a.uid = b.sid and b.subject = '数学'
LEFT JOIN scores c on a.uid = c.sid and c.subject = '语文'
LEFT JOIN scores d on a.uid = d.sid and d.subject = '英语'
image.png
WHERE
(CASE WHEN b.subject is null THEN 0 ELSE 1 END)
+ (CASE WHEN c.subject is null THEN 0 ELSE 1 END)
+ (CASE WHEN d.subject is null THEN 0 ELSE 1 END) >= 2
image.png
上面实现方式在条件增多的情况下语句过长可以使用
SELECT a.name FROM student a JOIN scores b ON a.uid = b.sid WHERE b.subject in ('英语','数学','语文')
GROUP BY a.name HAVING COUNT(*) >=2
但是这样只是过滤出姓名，具体是哪一科目还需要自己查。
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