Oracle多表连接与子查询
1：等值连接
     迪卡尔集连接
                  select ename, a.deptno as a_deptno,b.deptno as b_deptno ,b.dname as 部门
                  from emp a, dept b

     等值连接
                  select ename, a.deptno as a_deptno,b.deptno as b_deptno ,b.dname as 部门
                  from emp a, dept b
                  where a.deptno = b.deptno;


2：非等值连接，在emp表和salgrade表中查找员工的姓名，工资，等级，工资上线，工资下线
                 select ename as 姓名, sal as 工资, grade as 工资等级,losal as 工资上线，hisal as 工资下线
                 from emp, salgrade
                 where sal between losal and hisal;


3:外连
       右外连
                 select e.ename, d.dname, e.deptno
                 from emp e,dept d
                 where e.deptno = d.deptno(+);

        左外连与右外连相反


     以hr登陆查询表employees 和departments 表
                 select  first_name as 姓名, department_name as 部门名称,  d.department_id as 部门编号
                 from employees e, departments d
                 where e.department_id= d.department_id(+) ;


4:自连：在同一个表中查询每个员工及上司的工号和姓名
                 select a.empno  as 员工编号, a.ename as 员工姓名, a.mgr as 上司的员工编号, b.ename as 上司姓名
                  from emp a, emp b
                 where a.mgr = b.empno;




5：sql99
         交叉连接 cross join------相当于迪卡尔集
                  select e.ename,d.dname
                  from emp e cross join dept d;

         自然连接 natural join------相当等值连接
                  select e.ename, d.dname
                  from emp e natural join dept d;

         using子句-----使用同名列查询
              select e.ename, d.dname
              from emp e join dept d
        &nb