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
              using (deptno);

         on子句 ------当列名不同时用on子句
              用on查询两张表
                   select e.ename, d.dname
                    from emp e join dept d
                   on e.deptno = d.deptno;

      用on查询多张表
               select e.ename, d.dname
               from emp e join dept d
               on e.deptno = d.deptno
               join 第三个表
               on 列1 = 列2;

     内连接（Inner join）-------内连接只返回满足连接条件的数据
              select employee_id, last_name, salary, department_id, department_name
              from employees inner join departments using(department_id);

     左外连
              select employee_id, last_name, salary, department_id, department_name
              from employees left join departments using(department_id);
 
     右外连
              select employee_id, last_name, salary, department_id, department_name
              from employees right join departments using(department_id);

     满外连
              select employee_id, last_name, salary, department_id, department_name
              from employees full outer join departments using(department_id);

    
    子查询------------
             查找出工资比scott高的人
              select ename, sal from emp
              where sal>
              (select sal from emp where ename='SCOTT');
 
     查找那些人和scott相同职位的人
              select ename, job from emp
              where job=
               (select job from emp where ename='SCOTT')  
               and ename <> 'SCOTT';

     any的用法  < any意味着小于最大、> any大于最小
              select empno, ename, sal, job
              from emp 
              where sal<any(select sal from emp where job='SALESMAN');

     all的用法 < all：小于所有，即小于最小、> all：大于所有，即大于最大
              select empno, ename, sal, job
              from emp
              where sal<all(select sal from emp where job='SALESMAN');

 示例：
         //新建一张表
         create table test1(
          eid number(10),
          name varchar2(20),
          birth date,
          salary number(8,2)
         );

         //向表中插入数据
         insert into test1 values(1001,'张三','20-5月',2300);
         insert into test1 values(1002,'张三','01-6月',3000);

         //查询表中的数据
         select * from test1;

          //新建第二张表，在表中不插入任何数据
          create table test2(
          eid number(10),
          name varchar2(20),
          birth date,
          salary number(8,2)
         );
 
         //查询第二张表中的数据
         select * from test2;

 
         //使用merge进行数据合并
         merge into test2
         using test1
         on(test.eid = test2.eid)
         when matched then
          update set name = test1.name, birth = test1.birth, salary = test1.salary
         when not matched then
          insert(eid,name,birth,salary) values(test1.eid, test1.name, test1.birth, test1.salaru);

         select * from test2