12-Mysql資料庫----多表查詢_ZenDei技術網路在線

本節重點：多表連接查詢符合條件連接查詢子查詢準備工作：準備兩張表，部門表（department）、員工表（employee） ps:觀察兩張表，發現department表中id=203部門在employee中沒有對應的員工，發現employee中id=6的員工在department表中沒有對 ...

本節重點：

多表連接查詢
符合條件連接查詢
子查詢

準備工作：準備兩張表，部門表（department）、員工表（employee）

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),
('nvshen','male',18,200),
('xiaomage','female',18,204)
;

# 查看表結構和數據
mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.19 sec)

mysql> desc employee;
+--------+-----------------------+------+-----+---------+----------------+
| Field  | Type                  | Null | Key | Default | Extra          |
+--------+-----------------------+------+-----+---------+----------------+
| id     | int(11)               | NO   | PRI | NULL    | auto_increment |
| name   | varchar(20)           | YES  |     | NULL    |                |
| sex    | enum('male','female') | NO   |     | male    |                |
| age    | int(11)               | YES  |     | NULL    |                |
| dep_id | int(11)               | YES  |     | NULL    |                |
+--------+-----------------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)

mysql> select * from department;
+------+--------------+
| id   | name         |
+------+--------------+
|  200 | 技術         |
|  201 | 人力資源     |
|  202 | 銷售         |
|  203 | 運營         |
+------+--------------+
4 rows in set (0.02 sec)

mysql> select * from employee;
+----+----------+--------+------+--------+
| 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)

ps:觀察兩張表，發現department表中id=203部門在employee中沒有對應的員工，發現employee中id=6的員工在department表中沒有對應關係。

一、多表連接查詢

兩張表的準備工作已完成，比如現在我要查詢的員工信息以及該員工所在的部門。從該題中，我們看出既要查員工又要查該員工的部門，肯定要將兩張表進行連接查詢，多表連接查詢。

重點：外鏈接語法

語法：

SELECT 欄位列表
    FROM 表1 INNER|LEFT|RIGHT JOIN 表2
    ON 表1.欄位 = 表2.欄位;

（1）先看第一種情況交叉連接：不適用任何匹配條件。生成笛卡爾積（關於笛卡爾積的含義，大家百度自行補腦）。

mysql> select * from employee,department;
+----+----------+--------+------+--------+------+--------------+
| id | name     | sex    | age  | dep_id | id   | name         |
+----+----------+--------+------+--------+------+--------------+
|  1 | egon     | male   |   18 |    200 |  200 | 技術         |
|  1 | egon     | male   |   18 |    200 |  201 | 人力資源     |
|  1 | egon     | male   |   18 |    200 |  202 | 銷售         |
|  1 | egon     | male   |   18 |    200 |  203 | 運營         |
|  2 | alex     | female |   48 |    201 |  200 | 技術         |
|  2 | alex     | female |   48 |    201 |  201 | 人力資源     |
|  2 | alex     | female |   48 |    201 |  202 | 銷售         |
|  2 | alex     | female |   48 |    201 |  203 | 運營         |
|  3 | wupeiqi  | male   |   38 |    201 |  200 | 技術         |
|  3 | wupeiqi  | male   |   38 |    201 |  201 | 人力資源     |
|  3 | wupeiqi  | male   |   38 |    201 |  202 | 銷售         |
|  3 | wupeiqi  | male   |   38 |    201 |  203 | 運營         |
|  4 | yuanhao  | female |   28 |    202 |  200 | 技術         |
|  4 | yuanhao  | female |   28 |    202 |  201 | 人力資源     |
|  4 | yuanhao  | female |   28 |    202 |  202 | 銷售         |
|  4 | yuanhao  | female |   28 |    202 |  203 | 運營         |
|  5 | nvshen   | male   |   18 |    200 |  200 | 技術         |
|  5 | nvshen   | male   |   18 |    200 |  201 | 人力資源     |
|  5 | nvshen   | male   |   18 |    200 |  202 | 銷售         |
|  5 | nvshen   | male   |   18 |    200 |  203 | 運營         |
|  6 | xiaomage | female |   18 |    204 |  200 | 技術         |
|  6 | xiaomage | female |   18 |    204 |  201 | 人力資源     |
|  6 | xiaomage | female |   18 |    204 |  202 | 銷售         |
|  6 | xiaomage | female |   18 |    204 |  203 | 運營         |

（2）內連接：只連接匹配的行

#找兩張表共有的部分，相當於利用條件從笛卡爾積結果中篩選出了匹配的結果
#department沒有204這個部門，因而employee表中關於204這條員工信息沒有匹配出來
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   | 技術         |
|  2 | alex    |   48 | female | 人力資源     |
|  3 | wupeiqi |   38 | male   | 人力資源     |
|  4 | yuanhao |   28 | female | 銷售         |
|  5 | nvshen  |   18 | male   | 技術         |
+----+---------+------+--------+--------------+
5 rows in set (0.00 sec)

#上述sql等同於
mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;

（3）外鏈接之左連接：優先顯示左表全部記錄

#以左表為準，即找出所有員工信息，當然包括沒有部門的員工
#本質就是：在內連接的基礎上增加左邊有，右邊沒有的結果
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     | 技術         |
|  5 | nvshen   | 技術         |
|  2 | alex     | 人力資源     |
|  3 | wupeiqi  | 人力資源     |
|  4 | yuanhao  | 銷售         |
|  6 | xiaomage | NULL         |
+----+----------+--------------+
6 rows in set (0.00 sec)

（4）外鏈接之右連接：優先顯示右表全部記錄

#以右表為準，即找出所有部門信息，包括沒有員工的部門
#本質就是：在內連接的基礎上增加右邊有，左邊沒有的結果
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    | 技術         |
|    2 | alex    | 人力資源     |
|    3 | wupeiqi | 人力資源     |
|    4 | yuanhao | 銷售         |
|    5 | nvshen  | 技術         |
| NULL | NULL    | 運營         |
+------+---------+--------------+
6 rows in set (0.00 sec)

（5）全外連接：顯示左右兩個表全部記錄(瞭解)

#外連接：在內連接的基礎上增加左邊有右邊沒有的和右邊有左邊沒有的結果
#註意：mysql不支持全外連接 full JOIN
#強調：mysql可以使用此種方式間接實現全外連接
語法：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 | 技術         |
|    5 | nvshen   | 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 | xiaomage | female |   18 |    204 | NULL | NULL         |
| NULL | NULL     | NULL   | NULL |   NULL |  203 | 運營         |
+------+----------+--------+------+--------+------+--------------+
7 rows in set (0.01 sec)

#註意 union與union all的區別：union會去掉相同的紀錄

二、符合條件連接查詢

示例1：以內連接的方式查詢employee和department表，並且employee表中的age欄位值必須大於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：以內連接的方式查詢employee和department表，並且以age欄位的升序方式顯示。

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一個查詢語句中。
#2：內層查詢語句的查詢結果，可以為外層查詢語句提供查詢條件。
#3：子查詢中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等關鍵字
#4：還可以包含比較運算符：= 、 !=、> 、<等

例子：

（1）帶in關鍵字的子查詢

#查詢平均年齡在25歲以上的部門名
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
select name from department
    where id not in 
        (select dep_id from employee group by dep_id);

（2）帶比較運算符的子查詢

#比較運算符：=、!=、>、>=、<、<=、<>
#查詢大於所有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 |
+---------+------+

#查詢大於部門內平均年齡的員工名、年齡
思路：
      （1）先對員工表(employee)中的人員分組(group by)，查詢出dep_id以及平均年齡。
       (2)將查出的結果作為臨時表,再對根據臨時表的dep_id和employee的dep_id作為篩選條件將employee表和臨時表進行內連接。
       (3)最後再將employee員工的年齡是大於平均年齡的員工名字和年齡篩選。



mysql> 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 |

(3)帶EXISTS關鍵字的子查詢

#EXISTS關字鍵字表示存在。在使用EXISTS關鍵字時，內層查詢語句不返回查詢的記錄。而是返回一個真假值。True或False
#當返回True時，外層查詢語句將進行查詢；當返回值為False時，外層查詢語句不進行查詢
#department表中存在dept_id=203，Ture
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 |
+----+----------+--------+------+--------+
#department表中存在dept_id=205，False
mysql> select * from employee  where exists (select id from department where id=204);
Empty set (0.00 sec)

小練習：

查詢每個部門最新入職的那位員工

#創建表
create table employee(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum('male','female') not null default 'male', #大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, #一個部門一個屋子
depart_id int
);


#查看表結構
mysql> desc employee;
+--------------+-----------------------+------+-----+---------+----------------+
| Field        | Type                  | Null | Key | Default | Extra          |
+--------------+-----------------------+------+-----+---------+----------------+
| id           | int(11)               | NO   | PRI | NULL    | auto_increment |
| name         | varchar(20)           | NO   |     | NULL    |                |
| sex          | enum('male','female') | NO   |     | male    |                |
| age          | int(3) unsigned       | NO   |     | 28      |                |
| hire_date    | date                  | NO   |     | NULL    |                |
| post         | varchar(50)           | YES  |     | NULL    |                |
| post_comment | varchar(100)          | YES  |     | NULL    |                |
| salary       | double(15,2)          | YES  |     | NULL    |                |
| office       | int(11)               | YES  |     | NULL    |                |
| depart_id    | int(11)               | YES  |     | NULL    |                |
+--------------+-----------------------+------+-----+---------+----------------+

#插入記錄
#三個部門：教學，銷售，運營
insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
('egon','male',18,'20170301','老男孩駐沙河辦事處外交大使',7300.33,401,1), #以下是教學部
('alex','male',78,'20150302','teacher',1000000.31,401,1),
('wupeiqi','male',81,'20130305','teacher',8300,401,1),
('yuanhao','male',73,'20140701','teacher',3500,401,1),
('liwenzhou','male',28,'20121101','teacher',2100,401,1),
('jingliyang','female',18,'20110211','teacher',9000,401,1),
('jinxin','male',18,'19000301','teacher',30000,401,1),
('成龍','male',48,'20101111','teacher',10000,401,1),

('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是銷售部門
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),

('張野','male',28,'20160311','operation',10000.13,403,3), #以下是運營部門
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬銀','female',18,'20130311','operation',19000,403,3),
('程咬銅','male',18,'20150411','operation',18000,403,3),
('程咬鐵','female',18,'20140512','operation',17000,403,3)
;

select * from employee as t1
inner join
(select post,max(hire_date) as new_date from employee group by post) as t2
on t1.post=t2.post
where t1.hire_date=t2.new_date;

小練習答案