1.条件逻辑
select ename,sal,
case when sal <= 2000 then ‘up’
when sal>= 5000 then ‘over’
else ‘ok’
end as status
from emp
2.以指定顺序返回查询结果
select ename,job,sal
from emp
where deptno = 10
order by sal asc
升序是asc 降序是的desc
也可以用数字对应第几列
比如最后一行还可以改写为:order by 3 asc
3.排序时对null的处理
标记两种可能的取值,一种代表null,一种代表非null
eg.非null的升序排列,null放到最后面
select ename,sal,comm
from(
selcet ename,sal,comm
case when comm is null then 0 else 1 end as is_null
from emp
)x
order by is_nulldesc,comm
4.依据条件逻辑动态调整排序项
如果job等于salesman,就按照comm排序,否则,按照sal排序
select ename,sal ,job,comm
from emp
order by case when job = 'salesman' then comm else sal end