15-6 多表查詢

一 多表查詢
例子：
新建表並插入數據

#建表
create table department(
id int,
name varchar(20) 
);

create table employee(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);

#插入數據
insert into department values
(200,'技術'),
(201,'人力資源'),
(202,'銷售'),
(203,'運營');

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),
('liwenzhou','male',18,200),
('jingliyang','female',18,204)
;

View Code

 

1 交叉鏈接：不適用任何匹配條件。生成笛卡爾積

select * from employee,department;

2 內鏈接：只鏈接匹配的行  inner join

#找兩張表共有的部分，至關於利用條件從笛卡爾積結果中篩選出了正確的結果
#department沒有204這個部門，於是employee表中關於204這條員工信息沒有匹配出來

mysql> select * from employee inner join department on employee.dep_id=department.id;
+----+-----------+--------+------+--------+------+--------------+
| id | name      | sex    | age  | dep_id | id   | name         |
+----+-----------+--------+------+--------+------+--------------+
|  1 | egon      | male   |   18 |    200 |  200 | 技術         |
|  2 | alex      | female |   48 |    201 |  201 | 人力資源     |
|  3 | wupeiqi   | male   |   38 |    201 |  201 | 人力資源     |
|  4 | yuanhao   | female |   28 |    202 |  202 | 銷售         |
|  5 | liwenzhou | male   |   18 |    200 |  200 | 技術         |
+----+-----------+--------+------+--------+------+--------------+
5 rows in set (0.27 sec)

 

3 外連接之左鏈接：優先顯示左表所有記錄  left join

mysql> select * from employee left join department on employee.dep_id=department.id;
+----+------------+--------+------+--------+------+--------------+
| id | name       | sex    | age  | dep_id | id   | name         |
+----+------------+--------+------+--------+------+--------------+
|  1 | egon       | male   |   18 |    200 |  200 | 技術         |
|  5 | liwenzhou  | male   |   18 |    200 |  200 | 技術         |
|  2 | alex       | female |   48 |    201 |  201 | 人力資源     |
|  3 | wupeiqi    | male   |   38 |    201 |  201 | 人力資源     |
|  4 | yuanhao    | female |   28 |    202 |  202 | 銷售         |
|  6 | jingliyang | female |   18 |    204 | NULL | NULL         |
+----+------------+--------+------+--------+------+--------------+
6 rows in set (0.00 sec)

 

4 外連接之右鏈接：優先顯示右表所有記錄

#以右表爲準，即找出全部部門信息，包括沒有員工的部門
#本質就是：在內鏈接的基礎上增長右邊有左邊沒有的結果

mysql> 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 | 技術         |
|    2 | alex      | female |   48 |    201 |  201 | 人力資源     |
|    3 | wupeiqi   | male   |   38 |    201 |  201 | 人力資源     |
|    4 | yuanhao   | female |   28 |    202 |  202 | 銷售         |
|    5 | liwenzhou | male   |   18 |    200 |  200 | 技術         |
| NULL | NULL      | NULL   | NULL |   NULL |  203 | 運營         |
+------+-----------+--------+------+--------+------+--------------+
6 rows in set (0.00 sec)

 

5 全外鏈接：顯示左右兩個表所有記錄

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 | 技術         |
|    5 | liwenzhou  | male   |   18 |    200 |  200 | 技術         |
|    2 | alex       | female |   48 |    201 |  201 | 人力資源     |
|    3 | wupeiqi    | male   |   38 |    201 |  201 | 人力資源     |
|    4 | yuanhao    | female |   28 |    202 |  202 | 銷售         |
|    6 | jingliyang | female |   18 |    204 | NULL | NULL         |
| NULL | NULL       | NULL   | NULL |   NULL |  203 | 運營         |

+------+------------+------

 

全外鏈接：在內鏈接的基礎上增長左邊有右邊沒有的和右邊有左邊沒有的結果
#注意：mysql不支持全外鏈接 full JOIN
#強調：mysql能夠使用此種方式間接實現全外鏈接  

二 符合條件鏈接查詢

#示例1：之內鏈接的方式查詢employee和department表，而且employee表中的age字段值必須大於25,即找出年齡大於25歲的員工以及員工所在的部門
select employee.name,department.name from employee inner join department
    on employee.dep_id = department.id
    where age > 25;

#示例2：之內鏈接的方式查詢employee和department表，而且以age字段的升序方式顯示
select employee.id,employee.name,employee.age,department.name from employee,department
    where employee.dep_id = department.id
    and age > 25
    order by age asc;

 

   
三 子查詢
#1：子查詢是將一個查詢語句嵌套在另外一個查詢語句中。
#2：內層查詢語句的查詢結果，能夠爲外層查詢語句提供查詢條件。
#3：子查詢中能夠包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等關鍵字
#4：還能夠包含比較運算符：= 、 !=、> 、<等

1 帶IN關鍵字的子查詢

說明：括號裏面的語句是查出來大於25歲的部門id
mysql> select id,name from department where id in (select dep_id  from employee group by dep_id having avg(age) > 25);
+------+--------------+
| id   | name         |
+------+--------------+
|  201 | 人力資源     |
|  202 | 銷售         |
+------+--------------+
2 rows in set (0.31 sec)

#查看技術部員工姓名
select name from employee
    where dep_id in 
        (select id from department where name='技術');

#查看不足1人的部門名(子查詢獲得的是有人的部門id)
select name from department where id not in (select distinct dep_id from employee);

 

2 帶比較運算符的子查詢


#比較運算符：=、!=、>、>=、<、<=、<>
例子1 #查詢大於全部人平均年齡的員工名與年齡

mysql> select name,age from employee where age > (select avg(age) from employee);
+---------+------+
| name    | age  |
+---------+------+
| alex    |   48 |
| wupeiqi |   38 |
+---------+------+
2 rows in set (0.00 sec)

 

例子2：#查詢大於部門內平均年齡的員工名、年齡

mysql> select employee.name,employee.age from employee
    -> inner join
    -> (select dep_id,avg(age)as avg_age from employee group by dep_id) as dep
    -> on employee.dep_id=dep.dep_id
    -> where
    -> employee.age>dep.avg_age;
+------+------+
| name | age  |
+------+------+
| alex |   48 |
+------+------+
1 row in set (0.00 sec)

查詢最新入職的員工名字

mysql> select name,emp.hire_date,emp.post from emp inner join (select post,max(hire_date) as hire_date from emp group by post) as t2 on emp.post=t2.post where emp.hire_date=t2.hire_date;
+--------+------------+-----------------------------------------+
| name   | hire_date  | post                                    |
+--------+------------+-----------------------------------------+
| egon   | 2017-03-01 | 老男孩駐沙河辦事處外交大使              |
| alex   | 2015-03-02 | teacher                                 |
| 格格   | 2017-01-27 | sale                                    |
| 張野   | 2016-03-11 | operation                               |
+--------+------------+-----------------------------------------+
4 rows in set (0.00 sec)