--执行顺序 From Where Select 
select * from 
(select sal as salary,comm as commission from emp ) x where salary<5000 
--得出 Name Work as a Job 
select ename +' Work as a'+job as msg from emp where deptno=10 
--如果员工工资小于2000返回UnderPaid 大于等于4k 返回OverPaid 之间返回OK 
select ename,sal, 
case when sal<2000 then 'UnderPaid' 
when sal>=4000 then 'OverPaid' 
else 
'OK' 
end 
from emp 
--从表中随机返回N条记录 newid() 
--order by 字句中指定数字常量时，是要求根据select列表中相应位置的列排序 
--order by 字句中用函数时，则按函数在没一行计算结果排序 
select top 5 ename from emp order by newid() 
--找空值is null 
select * from emp where comm is null 
--将空值转换为实际值 
--解释：返回其参数中第一个非空表达式 
--coalesce 联合，合并，结合.英音：[,kəuə'les]美音：[,koə'lɛs] 
select coalesce(comm, 1),empNo from emp 
--按模式搜索 
--返回匹配特定子串或模式的行 
select ename,job 
from emp 
where deptno in(10,20) 
--按子串排序 按照职位字段的 最后两个字符排序 
select ename, job from emp order by substring(job,len(job)-2,2) 
--select top 2 len(job)-2 from emp 
--select top 2 job from emp 
--☆☆☆☆☆处理排序空值☆☆☆☆☆ [只能是大于0] 
select ename ,sal,comm 
from emp 
order by 1 desc 
-- 以降序或升序方式排序非空值，将空值放到最后，可以用case 
select ename,sal,comm from 
( 
select ename ,sal,comm , 
case when comm is null then 0 else 1 end as A 
from emp 
) x 
order by A desc ,comm desc