主要內容：
一、多表連接查詢
二、復合條件連接查詢
三、子查詢
1️⃣ 多表連接查詢
一、准備表
#建表
create table department( id int, name varchar(20) ); create table emp( 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 empl(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) ;
#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 | |
+-------+-------------+------+-----+---------+-------+ mysql> desc emp; +--------+-----------------------+------+-----+---------+----------------+
| 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 | |
+--------+-----------------------+------+-----+---------+----------------+ mysql> select * from department; +------+--------------+
| id | name |
+------+--------------+
| 200 | 技術 |
| 201 | 人力資源 |
| 202 | 銷售 |
| 203 | 運營 |
+------+--------------+ mysql> select * from emp; +----+------------+--------+------+--------+
| 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 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+
二、多表連接查詢
#重點：外鏈接語法
SELECT 字段列表 FROM 表1 INNER|LEFT|RIGHT JOIN 表2 ON 表1.字段 = 表2.字段;
1、交叉連接：不適用任何匹配條件。生成笛卡爾積
mysql> select * from emp,department;
連接結果：
+----+------------+--------+------+--------+------+--------------+
| 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 | liwenzhou | male | 18 | 200 | 200 | 技術 |
| 5 | liwenzhou | male | 18 | 200 | 201 | 人力資源 |
| 5 | liwenzhou | male | 18 | 200 | 202 | 銷售 |
| 5 | liwenzhou | male | 18 | 200 | 203 | 運營 |
| 6 | jingliyang | female | 18 | 204 | 200 | 技術 |
| 6 | jingliyang | female | 18 | 204 | 201 | 人力資源 |
| 6 | jingliyang | female | 18 | 204 | 202 | 銷售 |
| 6 | jingliyang | 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正確的結果 #department沒有204這個部門，因而emp表中關於204這條員工信息沒有匹配出來
mysql> select emp.id,emp.name,emp.age,emp.sex,department.name from emp inner join department on emp.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 | liwenzhou | 18 | male | 技術 |
+----+-----------+------+--------+--------------+
3、外鏈接之左連接：優先顯示左表全部記錄
#以左表為准，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 emp.id,emp.name,department.name as depart_name from emp left join department on emp.dep_id=department.id; +----+------------+--------------+
| id | name | depart_name |
+----+------------+--------------+
| 1 | egon | 技術 |
| 5 | liwenzhou | 技術 |
| 2 | alex | 人力資源 |
| 3 | wupeiqi | 人力資源 |
| 4 | yuanhao | 銷售 |
| 6 | jingliyang | NULL |
+----+------------+--------------+
4、外鏈接之右鏈接：優先顯示右表全部記錄
#以右表為准，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 emp.id,emp.name,department.name as depart_name from emp right join department on emp.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 | liwenzhou | 技術 |
| NULL | NULL | 運營 |
+------+-----------+--------------+
5、全外連接：顯示左右兩個表全部記錄
全外連接：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 #注意：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
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 |
+------+------------+--------+------+--------+------+--------------+
注意：union與union all 的區別：union會去掉相同的記錄
2️⃣ 復合條件連接查詢
#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：以內連接的方式查詢employee和department表，並且以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
3️⃣ 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
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 in (select dep_id from employee group by dep_id having count(id) <=1);
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 emp where age > (select avg(age) from emp); +---------+------+
| name | age |
+---------+------+
| alex | 48 |
| wupeiqi | 38 |
+---------+------+ rows in set (0.00 sec) #查詢大於部門內平均年齡的員工名、年齡
select t1.name,t1.age from emp t1 inner join (select dep_id,avg(age) avg_age from emp group by dep_id) t2 on t1.dep_id = t2.dep_id where t1.age > t2.avg_age;
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 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | 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)
練習：查詢每個部門最新入職的那位員工
company.employee
員工id id int
姓名 emp_name varchar
性別 sex enum
年齡 age int
入職日期 hire_date date
崗位 post varchar
職位描述 post_comment varchar
薪水 salary double
辦公室 office int
部門編號 depart_id int
#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 );
#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) ;
答案：
select * from employee as t1 inner join( select post,max(hire_date) as last_date from employee group by post ) as t2 on t1.post = t2.post where t1.hire_date = t2.last_date;
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+-----------------------------------------+------------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | post | last_date |
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+-----------------------------------------+------------+
| 1 | egon | male | 18 | 2017-03-01 | 老男孩駐沙河辦事處外交大使 | NULL | 7300.33 | 401 | 1 | 老男孩駐沙河辦事處外交大使 | 2017-03-01 |
| 2 | alex | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 | teacher | 2015-03-02 |
| 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 | sale | 2017-01-27 |
| 14 | 張野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 | operation | 2016-03-11 |
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+-----------------------------------------+------------+
4 rows in set (0.02 sec)