理解查询
服务器执行命令，在原始数据表中查找符合条件的数据，产生一个虚拟表。
虚拟表是数据组合后的重新展示，而不是原始的物理数据。
查询基本语法构成
select <列名>
from <表名>
[where <查询条件表达式>]
[order by <排序的列名> ASC或DESC]
查询全部行和列
select * from Students
查询部分行
select StudentName,Gender,出生日期
from Students where Gender='男' and Age>20
使用“AS”或使用“=”重新命名字段
select StudentName as 姓名,Gender as 性别,出生日期=birthday
from Students where Gender='男'
加号的使用
select 姓名=StudentName,地址和电话=StudentAddress+'【'+PhoneNumber+'】'
from Students where Gender='男'
select 总成绩=CSharp+SQLServerDB from ScoreList
注意：
1.+连接的数据类型必须兼容
2.如果使用+连接字符型数据，结果为字符串数据的连接
3.如果使用+连接数值型数据，结果为数值的和
查询空列
select * from ScoreList where SQLServerDB is null
使用常量列
select StudentName as 姓名,Gender as 性别,出生日期=birthday,所在学校='北京大学'
from Students where Gender='男'
限制固定行数
select top 5 StudentName,Gender,Birthday from Students
返回百分之多少行
select top 20 percent StudentName,Gender,Birthday from Students
按多列排序
select top 3 StudentId,CSharp as C#,DB=SQLServerDB
from ScoreList
where StudentId not in(select top 6 StudentId from ScoreList order by SQLServerDB DESC,CSharp DESC )
order by SQLServerDB DESC,CSharp DESC
模糊查询-like
select StudentName,StudentAddress from Students where StudentAddress like '天津%'
select StudentName,StudentAddress from Students where StudentName like '%小%'
模糊查询-between
select * from ScoreList where CSharp between 80 and 90
select StudentName,StudentAddress,Birthday from Students where Birthday between '1987-01-01' and '1988-01-01'
模糊查询-in
select StudentName,StudentAddress,age from Students where Age in(21,22,23)
select StudentName,StudentAddress,age from Students where StudentName in('王小虎','贺小张')
查询函数的使用
聚合函数（求和、统计、求最大值、最小值、平均值）
select SUM(CSharp) as C#总成绩 from ScoreList
select 总人数=COUNT(*) from Students
select MAX(Csharp) as C#最高分 ,MIN(CSharp) as C#最低分,AVG(CSharp) as C#平均分 from ScoreList
多表之间的数据查询
内连接（inner join...on...）查询
select Students.StudentId,C#成绩=CSharp,StudentName,ClassName
from ScoreList
inner join Students on Students.StudentId=ScoreList.StudentId
inner join StudentClass on Students.ClassId=StudentClass.ClassId
where CSharp >80
左外连接（left outer join...on...）
select Students.StudentId,StudentName,Gender ,C#成绩=CSharp from Students
left outer join ScoreList on Students.StudentId=ScoreList.StudentId
where Gender='男'
右外连接（right outer join...on...）
分组查询、统计及统计筛选（group by...having...）
select 班级=StudentClass.ClassName,人数=COUNT(*),C#最高分=Max(CSharp),DB最高分=MAX(SQLServerDB),
AVG(CSharp) as C#平均分,AVG(SQLServerDB) as DB平均分
from Students
inner Join StudentClass on Students.ClassId =StudentClass.ClassId
inner join ScoreList on ScoreList.StudentId=Students.StudentId
group by ClassName
having AVG(CSharp)>=70 and AVG(SQLServerDB)>=70
查询重复
--查询所有重复的记录
select * from ScoreList
where StudentId in(select StudentId from ScoreList group by StudentId having COUNT(*)>1)
order by StudentId
--其它方法
select * from ScoreList
where (select COUNT(*) from ScoreList s where s.StudentId=ScoreList.StudentId)>1
order by StudentId
分组查询对比
- where字句：从数据源中去掉不符合其搜索条件的数据。
- group by字句：搜集数据行到各个组中，统计函数为各个组计算统计值。
- having字句：在分组结果中，去掉不符合其组搜索条件的各组数据行。