1：笛卡乐积
展示的内容以第一个表为主（不变），第二个表的数据每行重复展示
select * from employee_tbl,user
想要展示公共字段必须加表名
select user.name,date,address,chinese from employee_tbl,user where employee_tbl.name=user.name
2：自连接（把表当做两张）
select worker.ename as '职员'，boss.ename as '上级名' from emp as worker，emp as boss where worker.mgr=boss.empno
3：子查询
单行子查询
select deptno from emp where deptno=(select deptno from emp where ename='smith')
多行子查询
select * from employee where deptNO in (select id from dept where deptName='明教')
-- 左联 left join on（左表为主--字段数据全部展示，没有的null展示）
select * from employee left join dept on dept.id=employee.deptNO
select employee_tbl.name,date,address from employee_tbl LEFT JOIN user on employee_tbl.name=user.name
标签：多表,name,查询,tbl,employee,where,select,user From： https://www.cnblogs.com/vip01/p/17062970.html