1中为什么from dual这个系统表？

--1

Select

      (Select Count(*) From emp)total,

      (Select Count(*) From emp Where to_char(hiredate,'yyyy')='1980') "1980",

      (Select Count(*) From emp Where to_char(hiredate,'yyyy')='1981') "1981",

      (Select Count(*) From emp Where to_char(hiredate,'yyyy')='1982') "1982",

      (Select Count(*) From emp Where to_char(hiredate,'yyyy')='1987') "1987"

From dual;

--2

Select um(Decode(to_char(hiredate,'yyyy'),'1980',1,0)) "1980",

       Sum(Decode(to_char(hiredate,'yyyy'),'1981',1,0))"1981",

       Sum(Count(*) totla ,

       SDecode(to_char(hiredate,'yyyy'),'1982',1,0)) "1982",

       Sum(Decode(to_char(hiredate,'yyyy'),'1987',1,0)) "1987"

       From emp ;