八.练习总结

练习一：找到员工表中工资最高的前三名，参考结果：

练习一参考结果:员工表中工资最高的前三名

-->答案：
SQL>select rownum, empno, ename, sal
    from (select * from emp order by sal desc)
    where rownum<=3;

=========================引申出分页的问题========================
--取8条数据，分成2页，现在取出第2页

SQL>select *
    from (select rownum r, e1.*
          from (select * from emp order by sal) e1
          where rownum<=8)
    where r>=5

练习二：找到员工表中薪水大于本部门平均薪水的员工。参考结果：

练习二参考结果:员工表中薪水大于本部门平均薪水的员工

-->答案：
SQL> select empno,ename,sal,avgsal
     from emp e, (select deptno,avg(sal) avgsal 
                  from emp 
                  group by deptno) d
     where e.deptno=d.deptno and e.sal> d.avgsal;

=========================引申出相关子查询========================
--相关子查询：将主查询中的某个值作为参数传递给子查询
--一般子查询会先查主查询再查子查询，但相关子查询例外，刚好相反

SQL> select empno, ename, sal, (select avg(sal) from emp where deptno=e.deptno) avgsal
     from emp e
     where sal>(select avg(sal) from emp where deptno=e.deptno);

练习三：统计每年入职的员工个数，参考结果：

练习三参考结果:统计每年入职的员工个数

-->答案：
SQL> select count(*) total,
            sum(decode(to_char(hiredate, 'yyyy'), '1980', 1)) "1980",
            sum(decode(to_char(hiredate, 'yyyy'), '1981', 1)) "1981",
            sum(decode(to_char(hiredate, 'yyyy'), '1982', 1)) "1982",
            sum(decode(to_char(hiredate, 'yyyy'), '1987', 1)) "1987"
     from emp;