数据库之多表查询
一、介绍
首先先准备表
员工表和部门表
#建表
create table department(
id int,
name varchar(20)
);
create table employee1(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);
#插入数据
insert into department values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');
insert into employee1(name,sex,age,dep_id) values
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('liwenzhou','male',18,200),
('jingliyang','female',18,204)
;
二、多表连接查询
1.交叉连接：不适用任何匹配条件。生成笛卡尔积、
select * from employee1 ,department;
2.内连接：找两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果。（只连接匹配的行）
#找两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果
#department没有204这个部门，因而employee表中关于204这条员工信息没有匹配出来
select * from employee1,department where employee1.dep_id=department.id;
#上面用where表示的可以用下面的内连接表示，建议使用下面的那种方法
select * from employee1 inner join department on employee1.dep_id=department.id;
#也可以这样表示哈
select employee1.id,employee1.name,employee1.age,employee1.sex,department.name from
employee1,department where employee1.dep_id=department.id;
3.左连接：优先显示左表全部记录。
#左链接：在按照on的条件取到两张表共同部分的基础上，保留左表的记录
select * from employee1 left join department on department.id=employee1.dep_id;
select * from department left join employee1 on department.id=employee1.dep_id;
4.右链接：优先显示右表全部记录。
#右链接：在按照on的条件取到两张表共同部分的基础上，保留右表的记录
select * from employee1 right join department on department.id=employee1.dep_id;
select * from department right join employee1 on department.id=employee1.dep_id;
5.全外连接：显示左右两个表的全部记录。
注意：mysql不支持全外连接 full join
强调：mysql可以使用union间接实现全外连接
select * from employee1 left join department on department.id=employee1.dep_id
union
select * from employee1 right join department on department.id=employee1.dep_id;
三、符合条件连接查询
示例1：以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25,
即找出公司所有部门中年龄大于25岁的员工
select * from employee1 inner join department on employee1.dep_id=department.id
and age>25;
示例2：以内连接的方式查询employee和department表，并且以age字段的升序方式显示
select * from employee1 inner join department on employee1.dep_id=department.id
and age>25 order by age asc;
四、子查询
#1：子查询是将一个查询语句嵌套在另一个查询语句中。
#2：内层查询语句的查询结果，可以为外层查询语句提供查询条件。
#3：子查询中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字
#4：还可以包含比较运算符：= 、 !=、> 、<等
小练习
#查询平均年龄在25岁以上的部门名
select name from department where id in (
select dep_id from employee group by dep_id having avg(age) > 25
);
#查看技术部员工姓名
select name from employee where dep_id = (select id from department where name='技术');
#查看小于2人的部门名
select name from department where id in (
select dep_id from employee1 group by dep_id having count(id) < 2
)
union
select name from department where id not in (select distinct dep_id from employee1);
#提取空部门 #有人的部门
select * from department where id not in (select distinct dep_id from employee1);
或者：
select name from department where id in
(
select dep_id from employee1 group by dep_id having count(id) < 2
union
select id from department where id not in (select distinct dep_id from employee1);
);
五、综合练习
一、SELECT语句关键字的定义顺序
1 SELECT DISTINCT <select_list>
2 FROM <left_table>
3 <join_type> JOIN <right_table>
4 ON <join_condition>
5 WHERE <where_condition>
6 GROUP BY <group_by_list>
7 HAVING <having_condition>
8 ORDER BY <order_by_condition>
9 LIMIT <limit_number>
二 、SELECT语句关键字的执行顺序
(7) SELECT
(8) DISTINCT <select_list>
(1) FROM <left_table>
(3) <join_type> JOIN <right_table>
(2) ON <join_condition>
(4) WHERE <where_condition>
(5) GROUP BY <group_by_list>
(6) HAVING <having_condition>
(9) ORDER BY <order_by_condition>
(10) LIMIT <limit_number>
从init.sql文件中导入数据
#准备表、记录
mysql> create database db1;
mysql> use db1;
mysql> source /root/init.sql
1 1、查询所有的课程的名称以及对应的任课老师姓名
2 select cname 课程名称,tname 老师姓名 from course left join teacher on course.teacher_id = teacher.tid;
3
4 2、查询学生表中男女生各有多少人
5 select gender ,count(gender) from student group by gender;
6
7 3、查询物理成绩等于100的学生的姓名
8 select sname from student inner join score on student.sid = score.student_id
9 join course on score.course_id = course.cid
10 where cname = '物理' and num = 100;
11
12 4、查询平均成绩大于八十分的同学的姓名和平均成绩
13 (1):
14 select sname 姓名,avg(num) 平均成绩 from student inner join score on student.sid = score.student_id
15 group by student_id having avg(num)>80;
16 (2):
17 select sname,avg(num) from student inner join score on student.sid = score.student_id
18 group by student_id having avg(num)>80
19
20 5、查询所有学生的学号，姓名，选课数，总成绩
21 select student.sid 学号,sname 姓名,count(course_id) 选课数,sum(num) 总成绩 from student left join score on student.sid = score.student_id
22 group by student.sid ;
23 6、 查询姓李老师的个数
24 select count(tid) from teacher where tname like '李%';
25
26 7、 查询没有报李平老师课的学生姓名
27 select sname 姓名 from student where sid not in (
28 select student_id from score where course_id in(
29 select cid from course where teacher_id =(
30 select tid from teacher where tname = '李平老师'
31 )));
32
33 8、 查询物理课程比生物课程高的学生的学号
34 select t1.student_id 学号 from
35 (select student_id ,num from score inner join course on score.course_id=course.cid where cname='物理' )as t1
36 inner join
37 (select student_id , num from score inner join course on score.course_id=course.cid where cname = '生物') as t2
38 on t1.student_id = t2.student_id
39 where t1.num>t2.num;
40
41
42
43 9、 查询没有同时选修物理课程和体育课程的学生姓名
44 (1):
45 select sname from student inner join score on student.sid = score.student_id
46 join course on course.cid=score.course_id and cname in ('物理','体育')
47 group by student_id having count(course_id)!=2;
48
49 (2):
50 select sname from student join score on student.sid = score.student_id
51 join course on course.cid=score.course_id where cname ='物理' or cname= '体育'
52 group by student_id having count(course_id) !=2;
53
54 10、查询挂科超过两门(包括两门)的学生姓名和班级
55 select sname 姓名,caption 班级 from student inner join score on student.sid = score.course_id
56 join class on class.cid = score.course_id
57 where num<60 group by student_id having count(course_id)>=2;
58
59 11、查询选修了所有课程的学生姓名
60 (1)
61 select sname 姓名 ,所有的课程数 from student inner join
62 (select student_id,count(course_id) 所有的课程数 from score group by student_id having count(course_id) = (
63 select count(cid) from course)) as t1
64 on t1.student_id = student.sid;
65
66 (2)
67 select sname,count(course_id) from student inner join score on student.sid = score.student_id
68 group by student_id having count(course_id)=(select count(cid) from course);
69
70 12、查询李平老师教的课程的所有成绩记录
71 (1):
72 select num from score inner join course on course.cid=score.course_id
73 join teacher on teacher.tid=course.teacher_id
74 where tname = '李平老师';
75
76 (2):
77 select num from score where course_id in(select cid from course where teacher_id=
78 (select tid from teacher where tname='李平老师'));
79
80 13、查询全部学生都选修了的课程号和课程名
81 select cid 课程号,cname 课程名 from course
82 select ;
83 14、查询每门课程被选修的次数
84 (1)
85 select course.cname,count(student_id) 选课人数 from score inner join course on score.course_id=course.cid
86 group by course_id;
87
88 (2):也可以按照name分组
89 select course.cname,count(student_id) 选课人数 from score inner join course on score.course_id=course.cid
90 group by cname;
91
92 15、查询之选修了一门课程的学生姓名和学号
93 select sname 姓名,student_id 学号 from student inner join score on student.sid = score.student_id
94 group by student_id having count(course_id)=1;
95
96 16、查询所有学生考出的成绩并按从高到低排序（成绩去重）
97 select distinct num from score order by num desc;
98
99 17、查询平均成绩大于85的学生姓名和平均成绩
100 (1):
101 select sname 姓名,avg(num) 平均成绩 from student inner join score on student.sid = score.student_id
102 group by student_id having avg(num)>85;
103
104 (2):
105 select student.sname,avg_num from student inner join
106 (select student_id,avg(num) as avg_num from score group by student_id having avg(num) > 85
107 ) t1
108 on student.sid=t1.student_id;
109
110
111 18、查询生物成绩不及格的学生姓名和对应生物分数
112 (1):
113 select student.sname ,num 生物成绩 from student inner join score on student.sid = score.student_id
114 join course on course.cid=score.course_id
115 where cname='生物' and num<60;
116
117 (2):
118 select student.sname,t1.num from student inner join
119 (
120 select student_id,num from score
121 where course_id=(select cid from course where cname='生物') and num < 60
122 ) t1
123 on t1.student_id=student.sid
124 ;
125
126 19、查询在所有选修了李平老师课程的学生中，这些课程(李平老师的课程，不是所有课程)平均成绩最高的学生姓名
127 select sname from student where sid=(
128 select student_id from score where course_id in (
129 select cid from course where teacher_id=(select tid from teacher where tname='李平老师')
130 ) group by student_id order by avg(num) desc limit 1
131 )
