原创: http://106.13.73.98/__/26/
目录
先准备两张表：部门表（department)、员工表（employee)
-
# 部门表
-
create
table department(
-
id
int primary
key auto_increment,
-
name
varchar(
20)
not
null
-
);
-
-
# 员工表
-
create
table employee(
-
id
int primary
key auto_increment,
-
name
varchar(
20)
not
null,
-
sex enum(
'male',
'female')
not
null
default
'male',
-
age
int
not
null,
-
dep_id
int
not
null
-
);
-
-
# 插入数据
-
insert
into department
values
-
(
200,
"技术"),
-
(
201,
"人力资源"),
-
(
202,
"销售"),
-
(
203,
"运营")
-
;
-
-
insert
into employee(
name, sex, age, dep_id)
values
-
(
'egon',
'male',
18,
200),
-
(
'alex',
'female',
48,
201),
-
(
'wupeiqi',
'male',
38,
201),
-
(
'yuanhao',
'female',
28,
202),
-
(
'nvshen',
'male',
18,
200),
-
(
'xiaomage',
'female',
18,
204)
-
;
-
-
# 注意：
-
department表中id=203的部门在employee中没有对应的员工
-
employee表中id=6的员工在department表中没有对应的部门
多表链接查询
-
# 外链接语法：
-
select 字段列表
-
from 表
1
inner|
left|
right
join 表
2
-
on 表
1.字段 = 表
2.字段;
笛卡尔积：
即交叉链接
不适用任何匹配条件，生成笛卡尔积（关于笛卡尔积的含义，请自行百度）
select * from employee, department; # 语法
内链接 inner join
只链接匹配的行
-
# 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匹配的结果
-
# 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
-
mysql>
select
-
-> employee.id,employee.name,employee.age,employee.sex,department.name
-
->
from employee
inner
join department
-
->
on employee.dep_id = department.id;
-
+
----+---------+-----+--------+--------------+
-
| id | name | age | sex | name |
-
+
----+---------+-----+--------+--------------+
-
| 1 | egon | 18 | male | 技术 |
-
| 2 | alex | 48 | female | 人力资源 |
-
| 3 | wupeiqi | 38 | male | 人力资源 |
-
| 4 | yuanhao | 28 | female | 销售 |
-
| 5 | nvshen | 18 | male | 技术 |
-
+
----+---------+-----+--------+--------------+
-
5 rows in
set (
0.00 sec)
-
-
# 上述
sql等同于：
-
mysql>
select
-
-> employee.id,employee.name,employee.age,employee.sex,department.name
-
->
from employee,department
-
->
where employee.dep_id=department.id;
外链接之左链接 left join
优先显示左表全部记录
-
# 以左表为准，即找出所有员工信息，当然包括没有部门的员工
-
# 本质就是：在内连接的基础上增加左边有，右边没有的结果
-
mysql>
select
-
-> employee.id,employee.name,
-
-> department.name
as depart_name
-
->
from employee
left
join department
-
->
on employee.dep_id = department.id;
-
+
----+----------+--------------+
-
| id | name | depart_name |
-
+
----+----------+--------------+
-
| 1 | egon | 技术 |
-
| 5 | nvshen | 技术 |
-
| 2 | alex | 人力资源 |
-
| 3 | wupeiqi | 人力资源 |
-
| 4 | yuanhao | 销售 |
-
| 6 | xiaomage | NULL |
-
+
----+----------+--------------+
-
6 rows in
set (
0.00 sec)
外链接之右链接 right join
优先显示右表全部记录
-
# 以右表为准，即找出所有部门信息，包括没有员工的部门
-
# 本质就是：在内连接的基础上增加右边有，左边没有的结果
-
mysql>
select
-
-> employee.id,employee.name,
-
-> department.name
as depart_name
-
->
from employee
right
join department
-
->
on employee.dep_id = department.id;
-
+
------+---------+--------------+
-
| id | name | depart_name |
-
+
------+---------+--------------+
-
| 1 | egon | 技术 |
-
| 2 | alex | 人力资源 |
-
| 3 | wupeiqi | 人力资源 |
-
| 4 | yuanhao | 销售 |
-
| 5 | nvshen | 技术 |
-
| NULL | NULL | 运营 |
-
+
------+---------+--------------+
-
6 rows in
set (
0.00 sec)
全外链接
显示左右两个表全部记录
-
# 外连接：在内连接的基础上增加左边有右边没有的和右边有左边没有的结果
-
# 注意：mysql不支持全外连接 full JOIN
-
# 强调：mysql可以使用此种方式间接实现全外连接
-
-
mysql>
select *
from employee
left
join department
-
->
on employee.dep_id = department.id
-
->
union all
-
->
select *
from employee
right
join department
-
->
on employee.dep_id = department.id;
-
-
mysql>
select *
from employee
left
join department
-
->
on employee.dep_id = department.id
-
->
union
-
->
select *
from employee
right
join department
-
->
on employee.dep_id = department.id;
-
+
------+----------+--------+------+--------+------+--------------+
-
| id | name | sex | age | dep_id | id | name |
-
+
------+----------+--------+------+--------+------+--------------+
-
| 1 | egon | male | 18 | 200 | 200 | 技术 |
-
| 5 | nvshen | male | 18 | 200 | 200 | 技术 |
-
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
-
| 3 | wupeiqi | male | 38 | 201 | 201 | 人力资源 |
-
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
-
| 6 | xiaomage | female | 18 | 204 | NULL | NULL |
-
| NULL | NULL | NULL | NULL | NULL | 203 | 运营 |
-
+
------+----------+--------+------+--------+------+--------------+
-
7 rows in
set (
0.00 sec)
-
-
# 注意
union与
union all的区别：
union会去掉相同的纪录
符合条件链接查询
-
# 示例1 以内链接的方式查询：找出年龄大于25岁的员工以及员工所在的部门
-
mysql>
select employee.name,employee.age,department.name
-
->
from employee
inner
join department
-
->
on employee.dep_id = department.id
-
->
where age >
25;
-
+
---------+-----+--------------+
-
| name | age | name |
-
+
---------+-----+--------------+
-
| alex | 48 | 人力资源 |
-
| wupeiqi | 38 | 人力资源 |
-
| yuanhao | 28 | 销售 |
-
+
---------+-----+--------------+
-
3 rows in
set (
0.00 sec)
-
-
-
# 示例
2 以内链接的方式查询：以age字段的升序方式显示
-
mysql>
select employee.name,employee.age,department.name
-
->
from employee
inner
join department
-
->
on employee.dep_id = department.id
-
->
order
by age
asc; # 升序排序
-
+
---------+-----+--------------+
-
| name | age | name |
-
+
---------+-----+--------------+
-
| egon | 18 | 技术 |
-
| nvshen | 18 | 技术 |
-
| yuanhao | 28 | 销售 |
-
| wupeiqi | 38 | 人力资源 |
-
| alex | 48 | 人力资源 |
-
+
---------+-----+--------------+
-
5 rows in
set (
0.00 sec)
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、not exists 等关键字.
4. 还可以包含比较运算符：=、 !=、>、< 等.
示例1：带in关键字的子查询
-
# 查询平均年龄在25以上的部门名
-
select
id,
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
id,
name
from employee
-
where dep_id
in
-
(
select
id
from department
where
name=
"技术");
-
-
# 查无人的部门名
-
select
name
from department
-
where
id
not
in
-
(
select dep_id
from employee);
示例2：带比较运算符的子查询
-
# 比较运算符：=、!=、>、>=、<、<=、<>
-
# 查询大于所有人平均年龄的员工名与年龄
-
select
name,age
from employee
-
where age > (
select
avg(age)
from employee);
-
-
# 查询大于部门内平均年龄的员工名、年龄
-
思路：
-
（1）先对员工表(employee)中的人员分组(group by)，查询出dep_id以及平均年龄。
-
(2)将查出的结果作为临时表,再根据临时表的dep_id和employee的dep_id作为筛选条件将employee表和临时表进行内连接。
-
(3)最后再将employee员工的年龄是大于平均年龄的员工名字和年龄筛选。
-
-
select t1.name,t1.age
from employee
as t1
-
inner
join
-
(
select dep_id,
avg(age)
as avg_age
from employee
group
by dep_id)
as t2
-
on t1.dep_id = t2.dep_id
-
where t1.age > t2.avg_age;
-
-
+
------+-----+
-
| name | age |
-
+
------+-----+
-
| alex | 48 |
-
+
------+-----+
-
1 row in
set (
0.00 sec)
示例3：带exists关键字的子查询
-
# exists关键字表示存在。在使用exists关键字时，内层查询语句不返回查询记录。而是返回一个真假值：True 或 False
-
# 当返回True时，外层查询语句将进行查询；当返回值为False时，外层查询语句不进行查询
-
-
# exists为True时：
-
mysql>
select *
from employee
where
exists (
select
id
from department
where
id=
200);
-
+
----+----------+--------+-----+--------+
-
| id | name | sex | age | dep_id |
-
+
----+----------+--------+-----+--------+
-
| 1 | egon | male | 18 | 200 |
-
| 2 | alex | female | 48 | 201 |
-
| 3 | wupeiqi | male | 38 | 201 |
-
| 4 | yuanhao | female | 28 | 202 |
-
| 5 | nvshen | male | 18 | 200 |
-
| 6 | xiaomage | female | 18 | 204 |
-
+
----+----------+--------+-----+--------+
-
6 rows in
set (
0.00 sec)
-
-
#
exists为
False时：
-
mysql>
select *
from employee
where
exists (
select
id
from department
where
id=
204);
-
Empty
set (
0.00 sec)