16 create view V

as

select name||' '||sal as data

from student --建立一个视图

select * from V--查询这个视图

17处理数据空值

select xh

from student order by 1 desc—空值排列

18 select xh

from student order by 1 desc nulls last--将空值置于最后排序

19 select id,xh,sal,

case when xh is null then 0 else 1 end as is_null

from student—查找为空的值并建立一个表查看

20根据数据项的值进行排序

select id,xh,sal,depo from student

order by case when depo='工程部' then xh else sal end --有工程部的按照xh排序否则按照工资排序