写一些简单的SQL

这是一个关于学生，成绩以及课程的三表相关联查，下面是我的表的详细信息：

名字不好听大家可以随便改.....

成绩表：

课程表：

学生表：

接下来就是咱们的sql条件查询了：

-- 1、用一条 SQL 语句查询学生表每门课都大于 80 分的学生姓名。（禁用not in）

SELECT TS.stuname FROM t_student ts LEFT JOIN t_chengji tc ON ts.id=tc.stuid LEFT JOIN t_kecheng tk on tc.kechengid=tk.kechengid GROUP BY TS.stuname HAVING MIN(TC.chengji)>80

-- 2、查询各科成绩前两名的记录
SELECT
   tc.chengji,
   tc.id,
   tc.kechengid,
   tc.stuid
FROM
   t_chengji tc
   LEFT JOIN t_chengji tc1 ON tc.id=tc1.id
   AND tc.chengji <= tc1.chengji
GROUP BY
   tc.chengji,
   tc.id,
   tc.kechengid,
   tc.stuid
HAVING
   COUNT(tc1.kechengid)<=2
ORDER BY
   tc.kechengid,
   tc.chengji DESC

-- 3.按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩
SELECT ts.id,TS.stuname,tc1.chengji 语文,tc2.chengji 数学,tc3.chengji 英语,平均值
FROM t_student ts
LEFT JOIN t_chengji tc1
ON ts.id=tc1.stuid and tc1.kechengid=1
LEFT JOIN t_chengji tc2
ON ts.id=tc2.stuid and tc2.kechengid=2
LEFT JOIN t_chengji tc3
ON ts.id=tc3.stuid and tc3.kechengid=3
LEFT JOIN (SELECT ts.id,avg(tc.chengji) as 平均值
FROM t_student ts
LEFT JOIN t_chengji tc
ON ts.id=tc.stuid
GROUP BY ts.id ) H
ON ts.id=H.id
ORDER BY 平均值 DESC

-- 4、查询没学过”语文”课程的学生姓名禁用not in
SELECT ts.stuname
FROM t_student ts
WHERE NOT EXISTS(
   SELECT tc.kechengid FROM t_chengji tc
   WHERE tc.kechengid=1
   and ts.id=tc.stuid
   )

-- 5、查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩
SELECT ts.id,ts.stuname,b.avg_chengji FROM t_student ts
JOIN (SELECT tc.stuid,AVG(tc.chengji)as avg_chengji FROM t_chengji tc GROUP BY tc.stuid)b ON ts.id=b.stuid
where ts.id IN
(SELECT tc.stuid FROM t_chengji tc WHERE tc.chengji<60 GROUP BY tc.stuid HAVING COUNT(tc.stuid)>=2)

-- 6、查询不同课程平均分从高到低显示
SELECT AVG(tc.chengji) 语文,AVG(tc1.chengji) 数学,AVG(tc2.chengji) 英语 FROM t_kecheng tk
LEFT JOIN t_chengji tc
ON tc.kechengid=tk.kechengid AND tk.kechengid=1
LEFT JOIN t_chengji tc1
ON tc1.kechengid=tk.kechengid AND tk.kechengid=2
LEFT JOIN t_chengji tc2
ON tc2.kechengid=tk.kechengid AND tk.kechengid=3
ORDER BY AVG(tc.chengji),AVG(tc1.chengji),AVG(tc2.chengji)DESC

-- 实现行转列
SELECT tc.stuid,
SUM(IF(tc.kechengid=1,tc.chengji,0)) as '语文',
SUM(IF(tc.kechengid=2,tc.chengji,0)) as '数学',
SUM(IF(tc.kechengid=3,tc.chengji,0)) as '英语'
FROM t_chengji tc
GROUP BY tc.stuid
ORDER BY tc.chengji DESC

热爱生活，热爱代码！

猜你喜欢