select * from UserInfor   --查找所有字段

select username,UserId from UserInfor -- 查找username,UserId两个字段

select top 2* from UserInfor where (Major=‘计算机‘ or Major=‘土木工程‘) and Sex=0 order by Age desc  --降序

select top 2* from UserInfor where major in(‘计算机‘,‘土木工程‘) and Sex=0 order by Age              --升序

select * from UserInfor where Age >  -- max() min() Avg()

(
select Avg(Age) from UserInfor
)  

select sum(Age) from UserInfor    -- 年龄总和

select count(*) as  UserCount from UserInfor where Major=‘计算机‘  -- count(*) 代表某一列有多少个， as 代表别名

select count(*) as UserCount,Major from UserInfor group by major -- 按照major计算分别有多少人

select count(*) as UserCount,Major from UserInfor group by Major having count(*) >2  -- 这里having是配合group by使用，这里不能用where

select * from UserInfor where RealName like ‘%张%‘ -- 模糊查询 “张%”以张开头；“%张”以张结尾；“%张%”包含张

select distinct realname from userinfor  -- 只显示列中不同的值,不重复的数据

select top 2 *  -- 分页语句
from 
(
 select row_number() over(order by userId) as rownumber,* from userinfor -- row_number() over找出行号, 以userid排序,别名 rownumber
) A
where rownumber > 2  -- >0第一页；>2第二页 ...


select UserID,Age,   -- case when 语句
case
when age>=20 and age<=24 then ‘大一‘
when age>=25 and age<=28 then ‘大二‘
else ‘大三‘
end as usergrade from userinfor


select * from UserInfor where age between 20 and 24  -- betwwen and的用法


select UserId  ,username, ISNULL (classname,‘四班‘) from UserInfor -- 判断classname字段有没有NULL，有NULL的设定值为‘四班‘

select year(getdate()) -- 获取当前时间的年 Month(getdate()) day(getdate())

select * from UserInfor where year(getdate())-year(Birthday )>25  -- 找出年龄大于25岁的员工信息

select DATEADD (yy,100,getdate()) -- yy代表年， 当前时间加上100年；mm代表月；dd代表天

select DATEDIFF (yy,getdate(),‘2018/12/31‘) -- yy代表年，当前时间距离2018/12/31多少年；mm代表月；dd代表天


select UI.userid,UI.username,UI.qq,UI.realname,SS.Scoreid,ss.chinesescore  --联合查询的三种方法，以左边为准，右边没有的用NULL补
from UserInfor UI left join stuscore SS on
UI.UserId =SS.UserId 

select UI.userid,UI.username,UI.qq,UI.realname,SS.Scoreid,ss.chinesescore  --联合查询的三种方法，以右边为准，左边没有的用NULL补
from UserInfor UI right join stuscore SS on
UI.UserId =SS.UserId 

select UI.userid,UI.username,UI.qq,UI.realname,SS.Scoreid,ss.chinesescore  --联合查询的三种方法，两边都有的值
from UserInfor UI inner join stuscore SS on
UI.UserId =SS.UserId 


select UIR.*,PP.MotherName,PP.fathername from
(select UI.userid,UI.username,UI.qq,UI.realname,SS.Scoreid,ss.chinesescore   --三个表的联合查询,超过四个表的联合查询,就是需求有问题,要用到缓存
from UserInfor UI inner join stuscore SS on
UI.UserId =SS.UserId  ) UIR inner join Parent PP on
UIR.UserId =pp.UserId 

insert into parent values(5,‘kk‘,‘‘)--主键是自增类型，不需要赋值，后面所以字段可以为空值，但是不能没有对应值

insert into parent(UserId ,FatherName ) values(6,‘ss‘) -- 添加某些列,其他补NULL

update parent set FatherName =‘GG‘,MotherName =‘HH‘ where UserId =6 --修改表

delete parent where Userid=5 or userid=6 --删除表

insert into CopyParent(userid,copymothername,copyfathername) select UserId,Mothername,fathername from Parent --表的复制

“增查改删”的语句