重點:
多表連線查詢
符合條件連線查詢
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 ); #插入資料 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) ; # 檢視錶結構和資料 mysql> desc department; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 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 | | +--------+-----------------------+------+-----+---------+----------------+ 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 | +------+--------------+ 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 | +----+----------+--------+------+--------+ rows in set (0.00 sec)
1.多表連線查詢
兩張表的準備工作已完成，比如現在我要查詢的員工資訊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線查詢，多表連線查詢。
重點：外連結語法
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線：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）內連線：只連線匹配的行
#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這條員工資訊沒有匹配出來 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 | +----+---------+------+--------+--------------+ 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）外連結之左連線：優先顯示左表全部記錄
#以左表為準，即找出所有員工資訊，當然包括沒有部門的員工 #本質就是：在內連線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 | +----+----------+--------------+ rows in set (0.00 sec)
（4） 外連結之右連線：優先顯示右表全部記錄
#以右表為準，即找出所有部門資訊，包括沒有員工的部門 #本質就是：在內連線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 | +------+---------+--------------+ rows in set (0.00 sec)
（5） 全外連線：顯示左右兩個表全部記錄(瞭解)
#外連線：在內連線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不支援全外連線 full JOIN #強調：mysql可以使用此種方式間接實現全外連線 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 | +------+----------+--------+------+--------+------+--------------+ rows in set (0.01 sec) #注意 union與union all的區別：union會去掉相同的紀錄
2.符合條件連線查詢
示例1：以內連線的方式查詢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：以內連線的方式查詢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;
3.子查詢
子查詢是將一個查詢語句巢狀在另一個查詢語句上
內層語句的查詢結果可以作為外層查詢語句的條件
子查詢中包含 In, not in, any, all, exists, not exists等關鍵字
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); # 檢視技術部員工姓名 select name from employee where dep_id in (select id from department where name='技術'); #檢視不足1人的部門名 select name from department where id not in (select dep_id from employee group by dep_id);
（2）帶比較運算子的子查詢
#比較運算子：=、!=、>、>=、<、<=、<> #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線。 (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)
練習:
查詢每個部門最新入職的員工
#建立表 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 ); #檢視錶結構 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) ;
答案:
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;