create table student(name char(10),gender int);
insert into student values("jack",1);
insert into student values("rose",0);create table student_man(name char(10),gender int);
insert into student_man select * from student where gender = 1;

select * from   table1,table2,......# 笛卡尔积查询的结果会出现大量的错误数据即,数据关联关系错误!
添加过滤条件 从表外键值 等于 主表的主键值# 并且会产生重复的字段信息  例如员工里的 部门编号  和 部门表的id字段
在select 后指定需要查询的字段名称 案例:
select  dept.name 部门 ,dept.id 部门编号,emp.name 姓名,emp.id 员工编号,sex from emp ,dept where dept.id = dept_id;

create table stu(id int primary key auto_increment,name char(10));create table tea(id int primary key auto_increment,name char(10));create table tsr(id int primary key auto_increment,t_id int,s_id int,
foreign key(s_id) references stu(id),
foreign key(t_id) references tea(id));insert into stu values(null,"张三"),(null,"李四");
insert into tea values(null,"egon"),(null,"wer");
insert into tsr values(null,1,1),(null,1,2),(null,2,2);#egon老师教过哪些人?
select tea.name,stu.name from tea join tsr join stu
on
tea.id = t_id and stu.id = s_id
where tea.name = "egon";# 子查询实现
select * from stu where id in (select s_id from tsr where t_id = (select id from tea where name = "egon"));

语法:
select * from  table1 inner join table2;
案例:
select * from  emp inner join dept where dept_id = dept.id;inner可以省略
select * from  emp join dept where dept_id = dept.id;

需求: 要查询所有员工以及其所属的部门信息
select * from emp left join dept on dept_id = dept.id;
注意: 在外连接查询中不能使用where 关键字 必须使用on专门来做表的对应关系

需求: 要查询所有部门以及其对应的员工信息
select * from emp right join dept on dept_id = dept.id;

需求:查询所有员工与所有部门的对应关系
select * from emp full join dept on dept_id = dept.id;注意:mysql不支持全外连接 我们可以将 左外连接查询的结果  和 右外连接查询的结果 做一个合并
select * from emp left join dept on dept_id = dept.id
union
select * from emp right join dept on dept_id = dept.id;union的用法:
select * from emp
union
select * from emp;# union将自动去除重复的记录
# union all 不去重复 select sex,name from emp
union
select * from dept;
# 注意  union 必须保证两个查询结果 列数相同  一般用在多个结果结构完全一致时