oracle decode的用法

需求：分别统计emp表中1980，1981，1982，1987年入职的同事的数量。

这里用decode很容易就解决了：

select sum(t.num_1980) as "1980",
       sum(t.num_1981) as "1981",
       sum(t.num_1982) as "1982",
       sum(t.num_1987) as "1987"
  from (select (decode(to_char(e.hiredate, 'yyyy'), '1980', 1, 0)) num_1980,
               (decode(to_char(e.hiredate, 'yyyy'), '1981', 1, 0)) num_1981,
               (decode(to_char(e.hiredate, 'yyyy'), '1982', 1, 0)) num_1982,
               (decode(to_char(e.hiredate, 'yyyy'), '1987', 1, 0)) num_1987
          from emp e) t

　　结果：

decode(val1, val2, val3, val4):若val1等于val2,取值val3；否则取值val4