Orcle基本语句（五）

 --分页查询---begin
 --sqlserver top
 --mysql limit
 --oracle rownum(伪列),oracle中伪列(rownum,rowid)

 --查询工资为前五的信息
 SELECT ROWNUM,tmp.* FROM (SELECT * FROM emp WHERE SAL IS NOT NULL ORDER BY sal DESC) tmp;
 --查询工资在第6条至第12条的信息
 --方法一：
 --1.查询在前12（包含第12条）的信息
 SELECT sal FROM (SELECT * FROM emp WHERE SAL IS NOT NULL ORDER BY sal DESC) WHERE ROWNUM <=12;
 --2.查询在前6（包含第六条）的信息
 SELECT sal FROM (SELECT * FROM emp WHERE SAL IS NOT NULL ORDER BY sal DESC) WHERE ROWNUM <=5;
 --3.
 SELECT ROWNUM, tmp.*
   FROM (SELECT * FROM emp WHERE sal IS NOT NULL ORDER BY sal DESC) tmp
  WHERE sal IN
        (SELECT sal
           FROM (SELECT * FROM emp WHERE sal IS NOT NULL ORDER BY sal DESC)
          WHERE rownum <= 12)
    AND sal NOT IN
        (SELECT sal
           FROM (SELECT * FROM emp WHERE sal IS NOT NULL ORDER BY sal DESC)
          WHERE rownum <= 5);
 --方法二：嵌套子查询
 SELECT tem.*
   FROM (SELECT rownum r, tmp.*
           FROM (SELECT * FROM emp WHERE sal IS NOT NULL ORDER BY sal DESC) tmp) tem
  WHERE tem.r BETWEEN 6 AND 12;

 --查询工资最高的第6条到第12条的员工信息
 --1.
 SELECT *
   FROM (SELECT rownum row_top ,tt.*
           FROM (SELECT e.*
                   FROM employees e
                  WHERE e.salary IS NOT NULL
                  ORDER BY salary DESC) tt)
  WHERE row_top BETWEEN 6 AND 12;
 --2. 效率较于1
 SELECT *
   FROM (SELECT rownum row_top, tt.*
           FROM (SELECT rownum rn, e.*
                   FROM employees e
                  WHERE e.salary IS NOT NULL
                  ORDER BY salary DESC) tt)
  WHERE row_top <= 12
    AND row_top >= 6;
 --3.效率较于2---推荐使用
 SELECT *
   FROM (SELECT rownum rn, tt.*
           FROM (SELECT *
                   FROM employees e
                  WHERE e.salary IS NOT NULL
                  ORDER BY salary DESC) tt
          WHERE rownum <= 12)
  WHERE rn >= 6

 --视图
 --查询全部的20号 部门的雇员信息（雇员编号，姓名，工作，雇佣日期）
 CREATE OR REPLACE VIEW view20 AS SELECT empno, ename, job, hiredate FROM emp WHERE deptno = 20;
 --创建视图v_dept：查询每个部门的编号，名称，员工人数，员工工资总和
 CREATE OR REPLACE VIEW v_dept AS
 SELECT e.deptno,d.dname,COUNT(e.empno) c, SUM(e.sal) s
   FROM emp e
   FULL JOIN dept d
     ON e.deptno = d.deptno
  GROUP BY e.deptno,d.dname;

 SELECT * FROM v_dept WHERE c >= 3;
 SELECT Count(*) from emp;