标签：def max 表示 creat 连表 order art 运营 none
一：介绍
主题：
多表连接查询
符合条件连接查询
子查询
准备表
#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), (‘liwenzhou‘,‘male‘,18,200), (‘jingliyang‘,‘female‘,18,204) ; #查看表结构和数据 mysql> desc department; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 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 | | +--------+-----------------------+------+-----+---------+----------------+ mysql> select * from department; +------+--------------+ | id | name | +------+--------------+ | 200 | 技术 | | 201 | 人力资源 | | 202 | 销售 | | 203 | 运营 | +------+--------------+ 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 | liwenzhou | male | 18 | 200 | | 6 | jingliyang | female | 18 | 204 | +----+------------+--------+------+--------+
二 多表连接查询
重点：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;
1、内连接：把两张表有对应关系的记录连接成一张虚拟表
select * from emp inner join dep on emp.dep_id = dep.id;
#应用：
select * from emp,dep where emp.dep_id = dep.id and dep.name = "技术"; # 不要用where做连表的活
select * from emp inner join dep on emp.dep_id = dep.id
where dep.name = "技术"
;
2、左连接：在内连接的基础上，保留左边没有对应关系的记录
select * from emp left join dep on emp.dep_id = dep.id;
3、右连接：在内连接的基础上，保留右边没有对应关系的记录
select * from emp right join dep on emp.dep_id = dep.id;
4、全连接：在内连接的基础上，保留左、右边没有对应关系的记录
select * from emp left join dep on emp.dep_id = dep.id
union
select * from emp right join dep on emp.dep_id = dep.id;
#补充：多表连接可以不断地与虚拟表连接
查找各部门最高工资 select t1.* from emp as t1 inner join (select post,max(salary) as ms from emp group by post) as t2 on t1.post = t2.post where t1.salary = t2.ms ;
三：符合条件连接查询
#示例1:以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25，即找到年龄大于25岁的员工以及员工所在的部门 select employee.name,department.name from employee inner join department on employee.dep_id = department.id where age > 25; #示例2:以内连接的方式查询employee 和department表，并且以age字段的升序方式显示 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 ;
四 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 和 NOT EXISTS等关键词 #4、还可以包含比较运算符：=、！=、 >、<等
1、带IN关键词的子查询
#子查询：把一个查询语句用括号括起来，当做另一条查询语句的条件去用，称之为子查询 select emp.name from emp inner join dep on emp.dep_id =dep.id where dep.name=‘技术‘; select name from emp where dep_id = (select id from department where name =‘技术‘);
#查询平均年龄在25岁以上的部门名字 select name from dep where id in (select dep_id from emp group by dep_id having avg(age)>25); #查看技术部员工姓名 select name from emp where dep_id in (select id from dep where name =‘技术‘); #查看不足1人的部门名（子查询得到的是有人的部门id） select name from dep where id not in （select distinct dep_id from employee); #查看每个部门最新入职的那位员工 select t1.id,t1.name,t1.post,t1.hire_date,t2.max_date from emp as t1 inner join (select post, max(hire_date) as max_date from emp group by post) as t2 on t1.post = t2.post where t1.hire_date = t2.max_date ;
2 带比较运算符的子查询
EXISTS关键词表示存在。在使用EXISTS关键词时，内层查询语句不返回查询的记录。
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；当返回为False时，外层查询语句不进行查询。
判断department表中存在dep_id=203，True 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 select * from employee where exists (select id from department where id=204);
标签：def max 表示 creat 连表 order art 运营 none
原文地址：https://www.cnblogs.com/wuchenyu/p/9021457.html
