继续玩，改变查询结果列名
select name as xm,birthday as workday from student;--修改列名name为xm，birthday为workday
select * from student;
select xh||sex from student--将选择的字段竖行排列
select name,sal,
case when sal<=10000 then 'underpaid'
when sal>=30000 then 'overpaid'
else 'ok'
end as status
from student--当大于30000，输出薪酬过高，小于10000输出过低，其他正常
select*from student where rownum<=5;--限制出现行数为五行¡
