sql 学习

表结构：

 CREATE TABLE [dbo].[student](
     [id] [int] IDENTITY(1,1) NOT NULL,
     [name] [nvarchar](100) NULL,
     [age] [int] NULL,
    [score] [int] NULL,
  CONSTRAINT [PK_mvc_test2] PRIMARY KEY CLUSTERED 
 (
     [id] ASC
 )WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
 ) ON [PRIMARY]
 
 GO

按年龄分组，查每个年龄的人数，而年龄要大于或等于19

 select [age],COUNT(1) '人数'
  from student 
  group by age
  having age >= 19

age	人数
19	2
20	1
21	1
33	1

 统计总分数

  select [name],[score] from student
  union all 
  select '总分',AVG([score]) from student

name	score
Tom	99
Jim	90
Rose	95
XiaoMing	97
Jake	100
Simth	0
总分	80

 按id排序，查出第5到第6条记录

 

  select top (2) * from student
   where [id] not in 
   (select top (4) [id] from student)

id	name	age	score
5	Jake	21	100
6	Simth	33	0

把一个表的数据复制到另一个表

insert into book2 (Title,Content,CreatTime)
(select top 200 Title,Content,CreatTime from book)

把一个表的数据更新到另一个表

update student set score=b.score
from student a JOIN  tem_student b ON a.id=b.id

sql  条件 case when then end

select [name],
(
case [score]
when 100 then '满分'
when 0 then '鸡蛋'
else CONVERT(varchar(50),[score])
end
) as 分数
from student

select [name],
(
case 
when [score]=100 then '满分'
when [score]=0 then '鸡蛋'
else CONVERT(varchar(50),[score])
end
) as 分数
from student

 查询表的结构 , 为什么不加这个查出来会多一个name,类型是sysname,   b.[name]<>'sysname'

SELECT a.[name] Name,b.[name] DataType,a.[length] Length,colstat IsPrimaryKey FROM syscolumns a
 left join systypes b on b.xtype = a.xusertype
 where a.[id] = object_id('student') and b.[name]<>'sysname' order by a.[name]

Name	DataType	Length	IsPrimaryKey
id	int	4	1
age	int	4	0
score	int	4	0
name	nvarchar	200	0

 --修改字段长度
alter table student alter column [name] varchar(50) 

--查询数据库连接数                               
SELECT * FROM 
[Master].[dbo].[SYSPROCESSES] WHERE [DBID] 
IN (SELECT  [DBID] FROM  [Master].[dbo].[SYSDATABASES] 
WHERE  NAME='student'
)