分析:平均 avg:GROUP BY分组
从高到低:ORDER BY
所有学生的所有课程的成绩:行转列
所有学生----外联(所有):RIGHT JOIN右联
SELECT
s.*,
c.cname,
t.tname,
sc.score
FROM
t_mysql_teacher t,
t_mysql_course c,
t_mysql_student s,
t_mysql_score sc
WHERE
t.tid = c.tid
AND c.cid = sc.cid
AND sc.sid = s.sid
AND t.tname = '张三'
分析: 没有学全
学全了有多少门:统计一共有多少门学科
统计每一个学生学了多少门
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( 1 ) FROM t_mysql_course)
分析:没学过: 子查询
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
分析:平均 GROUP BY
从高到低 ORDER BY
所有学生的所有课程的成绩
姓名 语文 数学 英语 平均成绩
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
分析: GROUP BY
大量使用函数
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>=90,1,0))/(SELECT count(1)
from
t_mysql_student)*100,2),'%') 优秀率,
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>=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>=60,1,0))/(SELECT count(1)
from t_mysql_student)*100,2),'%') 及格率
FROM
t_mysql_score sc
LEFT JOIN t_mysql_course c ON sc.cid = c.cid
GROUP BY
c.cid,
c.cname