sql 簡單查詢

1、加號的使用，如果是數值型，則數值相加，如果是字符串型，則字符串拼接

select PId + CategoryId from PhoneList

2、查詢空列

select * from PhoneList where Gender is null

3.常量列

select PId,PName,Gender,PhoneNumber,所在大學='天津大學' from PhoneList

4.限制固定行數

select top 5 StudentName,Gender,Birthday from Students

select top 20 percent StudentName,Gender,Birthday from Students