SQL语句案例

目录

一. 前言

二. 使用外连接、内连接和子查询进行解答

三. 思维导图

一. 前言

        在前面我们对外连接、内连接和子查询的使用有了一些了解，今天我们将继续更深入的进行学习。（这里缺少的八个题目在博主的前面博客有解答，大家可以移步前面一篇博客）

二. 使用外连接、内连接和子查询进行解答

SELECT
	s.*,
	c.cname,
	t.tname,
	sc.score
FROM
	t_mysql_teacher t,
	t_mysql_student s,
	t_mysql_course c,
	t_mysql_score sc 
WHERE
	t.tid = c.tid 
	AND c.cid = sc.cid 
	AND sc.sid = s.sid 
	AND tname = '张三'

SELECT
	s.sid,
	s.sname,
	count( sc.score ) n 
FROM
	t_mysql_student s
	LEFT JOIN t_mysql_score sc ON s.sid = sc.sid 
GROUP BY
	s.sid,
	s.sname 
HAVING
	n < (
	SELECT
		count(*) 
	FROM
		t_mysql_course 
	)

SELECT
	s.sid,
	s.sname 
FROM
	t_mysql_score sc,
	t_mysql_student s 
WHERE
	s.sid = sc.sid 
	AND sc.cid NOT IN 
    ( SELECT cid FROM t_mysql_course c, t_mysql_teacher t 
    WHERE c.tid = t.tid AND t.tname = '张三' ) 
GROUP BY
	s.sid,
	s.sname

SELECT
	s.sid,
	s.sname,
	avg( sc.score ) n 
FROM
	t_mysql_student s,
	t_mysql_score sc 
WHERE
	s.sid = sc.sid 
	AND sc.score < 60 
GROUP BY
	s.sid,
	s.sname

SELECT
	s.*,
	sc.score 
FROM
	t_mysql_student s,
	t_mysql_score sc 
WHERE
	s.sid = sc.sid 
	AND sc.cid = '01' 
	AND sc.score < 60 
ORDER BY
	sc.score DESC

SELECT
	s.sid,
	s.sname,
	sum(( CASE WHEN sc.cid = '01' THEN sc.score END )) 语文,
	sum(( CASE WHEN sc.cid = '02' THEN sc.score END )) 数学,
	sum(( CASE WHEN sc.cid = '03' THEN sc.score END )) 英语,
	ROUND( AVG(sc.score) , 2 ) 平均分
FROM t_mysql_score sc
RIGHT JOIN t_mysql_student s 
	ON sc.sid = s.sid 
GROUP BY
	s.sid,
	s.sname

SELECT
	s.sid,
	s.sname,
	sum( if( sc.cid = '01', sc.score, 0 )) 语文,
	sum( if( sc.cid = '02', sc.score, 0 )) 数学,
	sum( if( sc.cid = '03', sc.score, 0 )) 英语,
	ROUND( AVG(sc.score) , 2 ) 平均分
FROM t_mysql_score sc
RIGHT JOIN t_mysql_student s 
	ON sc.sid = s.sid 
GROUP BY
	s.sid,
	s.sname

SELECT
	c.cid,
	c.cname,
	count(sc.sid) 人数,
	max(sc.score) 最高分,
	min(sc.score) 最低分,
	ROUND(avg(sc.score),2) 平均分,
	CONCAT(ROUND(sum(if(sc.score>=60,1,0))/(SELECT COUNT(1) 
	from t_mysql_student)*100 ,2),'%') 及格率,
	CONCAT(ROUND(sum(if(sc.score>=70 and sc.score<80,1,0))/(SELECT COUNT(1) 
	from t_mysql_student)*100 ,2),'%') 中等率,
	CONCAT(ROUND(sum(if(sc.score>=80 and sc.score<90,1,0))/(SELECT COUNT(1) 
	from t_mysql_student)*100 ,2),'%') 优良率,
	CONCAT(ROUND(sum(if(sc.score>=90,1,0))/(SELECT COUNT(1) 
	from t_mysql_student)*100 ,2),'%') 优秀率
FROM
	t_mysql_score sc
	LEFT JOIN t_mysql_course c ON sc.cid = c.cid 
GROUP BY
	c.cid,
	c.cname

三. 思维导图