Left join：即左连接，是以左表为基础，根据ON后给出的两表的条件将两表连接起来。结果会将左表所有的查询信息列出，而右表只列出ON后条件与左表满足的部分。左连接全称为左外连接，是外连接的一种。
Right join：即右连接，是以右表为基础，根据ON后给出的两表的条件将两表连接起来。结果会将右表所有的查询信息列出，而左表只列出ON后条件与右表满足的部分。右连接全称为右外连接，是外连接的一种。
Inner join：即内连接，同时将两表作为参考对象，根据ON后给出的两表的条件将两表连接起来。结果则是两表同时满足ON后的条件的部分才会列出。
COUNT(*) 函数返回在给定的选择中被选的行数。
HAVING语句通常与GROUP BY语句联合使用，用来过滤由GROUP BY语句返回的记录集。HAVING语句的存在弥补了WHERE关键字不能与聚合函数联合使用的不足
1.自行添加测试数据
2.查询平均成绩大于60分的同学的学号和平均成绩；
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SELECT
t1.sid,
AVG
(t2.number)
FROM
student t1
LEFT
JOIN
score t2
ON
t1.sid=t2.student_id
GROUP
BY
t1.sid
HAVING
AVG
(t2.number)>60;
|
3.查询所有同学的学号、姓名、选课数、总成绩；
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SELECT
t1.sid,t1.sname,t2.temp1,t3.temp2
FROM
student t1,(
SELECT
student_id,
COUNT
(sid) temp1
FROM
score
GROUP
BY
student_id) t2,
(
SELECT
student_id,
SUM
(number) temp2
FROM
score
GROUP
BY
student_id) t3
WHERE
t1.sid=t2.student_id
AND
t1.sid=t3.student_id;
#较复杂，有难度
|
4.查询姓“李”的老师的个数；
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1
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3
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SELECT
COUNT
(tid)
FROM
teacher
WHERE
tname
LIKE
'李%'
;
|
5.查询没学过“叶平”老师课的同学的学号、姓名；
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SELECT
t1.`sid`,t1.`sname`
FROM
student t1
WHERE
t1.`sid`
NOT
IN
(
SELECT
student_id
FROM
score
WHERE
course_id
IN
(
SELECT
t2.cid
FROM
teacher t1
LEFT
JOIN
course t2
ON
t1.`tid`=t2.`teacher_id`
WHERE
t1.tname=
'叶平'
ORDER
BY
t2.`cid`));
#子句
SELECT
student_id
FROM
score
WHERE
course_id
IN
(
SELECT
t2.cid
FROM
teacher t1
LEFT
JOIN
course t2
ON
t1.`tid`=t2.`teacher_id`
WHERE
t1.tname=
'叶平'
ORDER
BY
t2.`cid`);<br>#较复杂
|
6.查询学过“001”并且也学过编号“002”课程的同学的学号、姓名；
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#方法1
SELECT
t1.`student_id`,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
course_id=1
UNION
ALL
SELECT
t1.`student_id`,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
course_id=2;
#方法2
SELECT
t1.sid,t1.sname
FROM
(
SELECT
t1.`student_id` sid,t2.`sname` sname
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
course_id=1
UNION
ALL
SELECT
t1.`student_id`,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
course_id=2) t1
GROUP
BY
t1.sid
HAVING
COUNT
(*)>1;
|
7.查询学过“叶平”老师所教的所有课的同学的学号、姓名；
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10
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SELECT
t1.`student_id`,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
t1.`course_id`
IN
(
SELECT
t2.cid
FROM
teacher t1
LEFT
JOIN
course t2
ON
t1.`tid`=t2.`teacher_id`
WHERE
t1.tname=
'叶平'
ORDER
BY
t2.`cid`);
#子句
SELECT
t2.cid
FROM
teacher t1
LEFT
JOIN
course t2
ON
t1.`tid`=t2.`teacher_id`
WHERE
t1.tname=
'叶平'
ORDER
BY
t2.`cid`;
|
8.查询课程编号“002”的成绩比课程编号“001”课程低的所有同学的学号、姓名；
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#子句1
SELECT
number
FROM
score
WHERE
course_id=2;
#子句2
SELECT
number
FROM
score t1
WHERE
t1.`course_id`=1;
#总语句
SELECT
t3.`sid`,t3.`sname`
FROM
(
SELECT
student_id,number
FROM
score
WHERE
course_id=2) t1,(
SELECT
student_id,number
FROM
score
WHERE
course_id=1) t2,student t3
WHERE
t1.student_id=t3.sid
AND
t2.student_id=t3.sid
AND
t1.number<t2.number;
|
9.查询有课程成绩小于60分的同学的学号、姓名；
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7
8
9
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#方法1
SELECT
DISTINCT
t1.student_id,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
t1.number<60;
#方法2
SELECT
t1.sid,t1.sname
FROM
student t1,(
SELECT
student_id
FROM
score
WHERE
number<60
GROUP
BY
student_id) t2
WHERE
t1.sid=t2.student_id;
|
10.查询没有学全所有课的同学的学号、姓名；
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#总语句
SELECT
sid,sname
FROM
student
WHERE
sid
NOT
IN
(
SELECT
student_id
FROM
score
GROUP
BY
student_id
HAVING
COUNT
(course_id)=(
SELECT
COUNT
(cid)
FROM
course));
#子句
SELECT
sid,sname
FROM
student
WHERE
sid
NOT
IN
();
#子句：学全了的学生编号
SELECT
student_id
FROM
score
GROUP
BY
student_id
HAVING
COUNT
(course_id)=(
SELECT
COUNT
(cid)
FROM
course);
#子句：课程的总数
SELECT
COUNT
(cid)
FROM
course;
|
11.查询至少有一门课与学号为“001”的同学所学相同的同学的学号和姓名；
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1
2
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4
5
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11
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#总语句
SELECT
DISTINCT
t1.student_id,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
course_id
IN
(
SELECT
course_id
FROM
score
WHERE
student_id=1);
#子句:学号1所选所有课程
SELECT
course_id
FROM
score
WHERE
student_id=1
|
首先还是介绍一下这三个的定义
Left join：即左连接，是以左表为基础，根据ON后给出的两表的条件将两表连接起来。结果会将左表所有的查询信息列出，而右表只列出ON后条件与左表满足的部分。左连接全称为左外连接，是外连接的一种。
Right join：即右连接，是以右表为基础，根据ON后给出的两表的条件将两表连接起来。结果会将右表所有的查询信息列出，而左表只列出ON后条件与右表满足的部分。右连接全称为右外连接，是外连接的一种。
Inner join：即内连接，同时将两表作为参考对象，根据ON后给出的两表的条件将两表连接起来。结果则是两表同时满足ON后的条件的部分才会列出。
MySQL中没有全外连接，所以这里没做解释了。
1.自行添加测试数据
2.查询平均成绩大于60分的同学的学号和平均成绩；
|
1
2
3
|
SELECT
t1.sid,
AVG
(t2.number)
FROM
student t1
LEFT
JOIN
score t2
ON
t1.sid=t2.student_id
GROUP
BY
t1.sid
HAVING
AVG
(t2.number)>60;
|
3.查询所有同学的学号、姓名、选课数、总成绩；
|
1
2
3
4
5
|
SELECT
t1.sid,t1.sname,t2.temp1,t3.temp2
FROM
student t1,(
SELECT
student_id,
COUNT
(sid) temp1
FROM
score
GROUP
BY
student_id) t2,
(
SELECT
student_id,
SUM
(number) temp2
FROM
score
GROUP
BY
student_id) t3
WHERE
t1.sid=t2.student_id
AND
t1.sid=t3.student_id;
#较复杂，有难度
|
4.查询姓“李”的老师的个数；
|
1
2
3
|
SELECT
COUNT
(tid)
FROM
teacher
WHERE
tname
LIKE
'李%'
;
|
5.查询没学过“叶平”老师课的同学的学号、姓名；
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
|
SELECT
t1.`sid`,t1.`sname`
FROM
student t1
WHERE
t1.`sid`
NOT
IN
(
SELECT
student_id
FROM
score
WHERE
course_id
IN
(
SELECT
t2.cid
FROM
teacher t1
LEFT
JOIN
course t2
ON
t1.`tid`=t2.`teacher_id`
WHERE
t1.tname=
'叶平'
ORDER
BY
t2.`cid`));
#子句
SELECT
student_id
FROM
score
WHERE
course_id
IN
(
SELECT
t2.cid
FROM
teacher t1
LEFT
JOIN
course t2
ON
t1.`tid`=t2.`teacher_id`
WHERE
t1.tname=
'叶平'
ORDER
BY
t2.`cid`);<br>#较复杂
|
6.查询学过“001”并且也学过编号“002”课程的同学的学号、姓名；
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
|
#方法1
SELECT
t1.`student_id`,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
course_id=1
UNION
ALL
SELECT
t1.`student_id`,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
course_id=2;
#方法2
SELECT
t1.sid,t1.sname
FROM
(
SELECT
t1.`student_id` sid,t2.`sname` sname
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
course_id=1
UNION
ALL
SELECT
t1.`student_id`,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
course_id=2) t1
GROUP
BY
t1.sid
HAVING
COUNT
(*)>1;
|
7.查询学过“叶平”老师所教的所有课的同学的学号、姓名；
|
1
2
3
4
5
6
7
8
9
10
|
SELECT
t1.`student_id`,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
t1.`course_id`
IN
(
SELECT
t2.cid
FROM
teacher t1
LEFT
JOIN
course t2
ON
t1.`tid`=t2.`teacher_id`
WHERE
t1.tname=
'叶平'
ORDER
BY
t2.`cid`);
#子句
SELECT
t2.cid
FROM
teacher t1
LEFT
JOIN
course t2
ON
t1.`tid`=t2.`teacher_id`
WHERE
t1.tname=
'叶平'
ORDER
BY
t2.`cid`;
|
8.查询课程编号“002”的成绩比课程编号“001”课程低的所有同学的学号、姓名；
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
|
#子句1
SELECT
number
FROM
score
WHERE
course_id=2;
#子句2
SELECT
number
FROM
score t1
WHERE
t1.`course_id`=1;
#总语句
SELECT
t3.`sid`,t3.`sname`
FROM
(
SELECT
student_id,number
FROM
score
WHERE
course_id=2) t1,(
SELECT
student_id,number
FROM
score
WHERE
course_id=1) t2,student t3
WHERE
t1.student_id=t3.sid
AND
t2.student_id=t3.sid
AND
t1.number<t2.number;
|
9.查询有课程成绩小于60分的同学的学号、姓名；
|
1
2
3
4
5
6
7
8
9
|
#方法1
SELECT
DISTINCT
t1.student_id,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
t1.number<60;
#方法2
SELECT
t1.sid,t1.sname
FROM
student t1,(
SELECT
student_id
FROM
score
WHERE
number<60
GROUP
BY
student_id) t2
WHERE
t1.sid=t2.student_id;
|
10.查询没有学全所有课的同学的学号、姓名；
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
|
#总语句
SELECT
sid,sname
FROM
student
WHERE
sid
NOT
IN
(
SELECT
student_id
FROM
score
GROUP
BY
student_id
HAVING
COUNT
(course_id)=(
SELECT
COUNT
(cid)
FROM
course));
#子句
SELECT
sid,sname
FROM
student
WHERE
sid
NOT
IN
();
#子句：学全了的学生编号
SELECT
student_id
FROM
score
GROUP
BY
student_id
HAVING
COUNT
(course_id)=(
SELECT
COUNT
(cid)
FROM
course);
#子句：课程的总数
SELECT
COUNT
(cid)
FROM
course;
|
11.查询至少有一门课与学号为“001”的同学所学相同的同学的学号和姓名；
|
1
2
3
4
5
6
7
8
9
10
11
|
#总语句
SELECT
DISTINCT
t1.student_id,t2.`sname`
FROM
score t1
LEFT
JOIN
student t2
ON
t1.`student_id`=t2.`sid`
WHERE
course_id
IN
(
SELECT
course_id
FROM
score
WHERE
student_id=1);
#子句:学号1所选所有课程
SELECT
course_id
FROM
score
WHERE
student_id=1
|