select * from tb t
where not exists(select 1 from tb where stuNo=t.stuNo and stuName=stuName and [no] < t.no)
-------------------------------
请解述一下上面的语句的意思,谢谢。
not exists起什么作用。
数学1 起什么作用。等等。
where not exists(select 1 from tb where stuNo=t.stuNo and stuName=stuName and [no] < t.no)
-------------------------------
请解述一下上面的语句的意思,谢谢。
not exists起什么作用。
数学1 起什么作用。等等。
not exists (sql 不返回结果集为真) 如下:
表A
ID NAME
1 A1
2 A2
3 A3 表B
ID AID NAME
1 1 B1
2 2 B2
3 2 B3 表A和表B是一对多的关系 A.ID --> B.AID SELECT ID , NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE A.ID = B.AID)
执行结果为
1 A1
2 A2
原因可以按照如下分析
SELECT ID , NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.AID = 1)
-->SELECT * FROM B WHERE B.AID = 1有值返回真所以有数据 SELECT ID , NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.AID = 2)
-->SELECT * FROM B WHERE B.AID = 2有值返回真所以有数据 SELECT ID , NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.AID = 3)
-->SELECT * FROM B WHERE B.AID = 3无值返回真所以没有数据 NOT EXISTS 就是反过来
SELECT ID , NAME FROM A WHERE NOT EXIST (SELECT * FROM B WHERE A.ID = B.AID)
执行结果为
3 A3 =========================================================================== EXISTS = IN,意思相同不过语法上有点点区别,好像使用IN效率要差点,应该是不会执行索引的原因
SELECT ID , NAME FROM A WHERE ID IN (SELECT AID FROM B) NOT EXISTS = NOT IN ,意思相同不过语法上有点点区别
SELECT ID , NAME FROM A WHERE ID NOT IN (SELECT AID FROM B)
下面是普通的用法: SQL中IN,NOT IN,EXISTS,NOT EXISTS的用法和差别:
IN:确定给定的值是否与子查询或列表中的值相匹配。
IN 关键字使您得以选择与列表中的任意一个值匹配的行。
当要获得居住在 California、Indiana 或 Maryland 州的所有作者的姓名和州的列表时,就需要下列查询:
SELECT ProductID , ProductName FROM Northwind.dbo.Products WHERE CategoryID = 1 OR CategoryID = 4 OR CategoryID = 5
然而,如果使用 IN,少键入一些字符也可以得到同样的结果:
SELECT ProductID , ProductName FROM Northwind.dbo.Products WHERE CategoryID IN (1 , 4 , 5)
IN 关键字之后的项目必须用逗号隔开,并且括在括号中。
下列查询在 titleauthor 表中查找在任一种书中得到的版税少于 50% 的所有作者的 au_id,然后从 authors 表中选择 au_id 与
titleauthor 查询结果匹配的所有作者的姓名:
SELECT au_lname , au_fname FROM authors WHERE au_id IN (SELECT au_id FROM titleauthor WHERE royaltyper < 50)
结果显示有一些作者属于少于 50% 的一类。
NOT IN:通过 NOT IN 关键字引入的子查询也返回一列零值或更多值。
以下查询查找没有出版过商业书籍的出版商的名称。
SELECT pub_name FROM publishers WHERE pub_id NOT IN (SELECT pub_id FROM titles WHERE type = 'business')
使用 EXISTS 和 NOT EXISTS 引入的子查询可用于两种集合原理的操作:交集与差集。
两个集合的交集包含同时属于两个原集合的所有元素。
差集包含只属于两个集合中的第一个集合的元素。
EXISTS:指定一个子查询,检测行的存在。
本示例所示查询查找由位于以字母 B 开头的城市中的任一出版商出版的书名:
SELECT DISTINCT pub_name FROM publishers WHERE EXISTS (SELECT * FROM titles WHERE pub_id = publishers.pub_id AND type = 'business')
SELECT distinct pub_name FROM publishers WHERE pub_id IN (SELECT pub_id FROM titles WHERE type = 'business')
两者的区别:
EXISTS:后面可以是整句的查询语句如:SELECT * FROM titles
IN:后面只能是对单列:SELECT pub_id FROM titles
NOT EXISTS:
例如,要查找不出版商业书籍的出版商的名称:
SELECT pub_name FROM publishers WHERE NOT EXISTS (SELECT * FROM titles WHERE pub_id = publishers.pub_id AND type = 'business')
下面的查询查找已经不销售的书的名称:
SELECT title FROM titles WHERE NOT EXISTS (SELECT title_id FROM sales WHERE title_id = titles.title_id) 语法 EXISTS subquery
参数
subquery:是一个受限的 SELECT 语句 (不允许有 COMPUTE 子句和 INTO 关键字)。有关更多信息,请参见 SELECT 中有关子查询的讨论。 结果类型:Boolean 结果值:如果子查询包含行,则返回 TRUE。 示例
A. 在子查询中使用 NULL 仍然返回结果集 这个例子在子查询中指定 NULL,并返回结果集,通过使用 EXISTS 仍取值为 TRUE。 USE Northwind
GO
SELECT CategoryName
FROM Categories
WHERE EXISTS (SELECT NULL)
ORDER BY CategoryName ASC
GO B. 比较使用 EXISTS 和 IN 的查询 这个例子比较了两个语义类似的查询。第一个查询使用 EXISTS 而第二个查询使用 IN。注意两个查询返回相同的信息。 USE pubs
GO
SELECT DISTINCT pub_name
FROM publishers
WHERE EXISTS
(SELECT *
FROM titles
WHERE pub_id = publishers.pub_id
AND type = \'business\')
GO -- Or, using the IN clause: USE pubs
GO
SELECT distinct pub_name
FROM publishers
WHERE pub_id IN
(SELECT pub_id
FROM titles
WHERE type = \'business\')
GO
下面是任一查询的结果集: pub_name
----------------------------------------
Algodata Infosystems
New Moon Books C.比较使用 EXISTS 和 = ANY 的查询 本示例显示查找与出版商住在同一城市中的作者的两种查询方法:第一种方法使用 = ANY,第二种方法使用 EXISTS。注意这两种方法返回相同的信息。 USE pubs
GO
SELECT au_lname, au_fname
FROM authors
WHERE exists
(SELECT *
FROM publishers
WHERE authors.city = publishers.city)
GO -- Or, using = ANY USE pubs
GO
SELECT au_lname, au_fname
FROM authors
WHERE city = ANY
(SELECT city
FROM publishers)
GO
D.比较使用 EXISTS 和 IN 的查询 本示例所示查询查找由位于以字母 B 开头的城市中的任一出版商出版的书名: USE pubs
GO
SELECT title
FROM titles
WHERE EXISTS
(SELECT *
FROM publishers
WHERE pub_id = titles.pub_id
AND city LIKE \'B%\')
GO -- Or, using IN: USE pubs
GO
SELECT title
FROM titles
WHERE pub_id IN
(SELECT pub_id
FROM publishers
WHERE city LIKE \'B%\')
GO
E. 使用 NOT EXISTS NOT EXISTS 的作用与 EXISTS 正相反。如果子查询没有返回行,则满足 NOT EXISTS 中的 WHERE 子句。本示例查找不出版商业书籍的出版商的名称: USE pubs
GO
SELECT pub_name
FROM publishers
WHERE NOT EXISTS
(SELECT *
FROM titles
WHERE pub_id = publishers.pub_id
AND type = \'business\')
ORDER BY pub_name
GO
/******************************************************************************************************************************************************
1、Num、Name相同的重复值记录,没有大小关系只保留一条
2、Name相同,ID有大小关系时,保留大或小其中一个记录
整理人:中国风(Roy)日期:2008.06.06
******************************************************************************************************************************************************/--1、用于查询重复处理记录(如果列没有大小关系时2000用生成自增列和临时表处理,SQL2005用row_number函数处理)--> --> (Roy)生成測試數據
if not object_id('Tempdb..#T') is null
drop table #T
Go
Create table #T([ID] int,[Name] nvarchar(1),[Memo] nvarchar(2))
Insert #T
select 1,N'A',N'A1' union all
select 2,N'A',N'A2' union all
select 3,N'A',N'A3' union all
select 4,N'B',N'B1' union all
select 5,N'B',N'B2'
Go
--I、Name相同ID最小的记录(推荐用1,2,3),方法3在SQl05时,效率高于1、2
方法1:
Select * from #T a where not exists(select 1 from #T where Name=a.Name and ID<a.ID)方法2:
select a.* from #T a join (select min(ID)ID,Name from #T group by Name) b on a.Name=b.Name and a.ID=b.ID方法3:
select * from #T a where ID=(select min(ID) from #T where Name=a.Name)方法4:
select a.* from #T a join #T b on a.Name=b.Name and a.ID>=b.ID group by a.ID,a.Name,a.Memo having count(1)=1 方法5:
select * from #T a group by ID,Name,Memo having ID=(select min(ID)from #T where Name=a.Name)方法6:
select * from #T a where (select count(1) from #T where Name=a.Name and ID<a.ID)=0方法7:
select * from #T a where ID=(select top 1 ID from #T where Name=a.name order by ID)方法8:
select * from #T a where ID!>all(select ID from #T where Name=a.Name)方法9(注:ID为唯一时可用):
select * from #T a where ID in(select min(ID) from #T group by Name)--SQL2005:方法10:
select ID,Name,Memo from (select *,min(ID)over(partition by Name) as MinID from #T a)T where ID=MinID方法11:select ID,Name,Memo from (select *,row_number()over(partition by Name order by ID) as MinID from #T a)T where MinID=1生成结果:
/*
ID Name Memo
----------- ---- ----
1 A A1
4 B B1(2 行受影响)
*/
--II、Name相同ID最大的记录,与min相反:
方法1:
Select * from #T a where not exists(select 1 from #T where Name=a.Name and ID>a.ID)方法2:
select a.* from #T a join (select max(ID)ID,Name from #T group by Name) b on a.Name=b.Name and a.ID=b.ID order by ID方法3:
select * from #T a where ID=(select max(ID) from #T where Name=a.Name) order by ID方法4:
select a.* from #T a join #T b on a.Name=b.Name and a.ID<=b.ID group by a.ID,a.Name,a.Memo having count(1)=1 方法5:
select * from #T a group by ID,Name,Memo having ID=(select max(ID)from #T where Name=a.Name)方法6:
select * from #T a where (select count(1) from #T where Name=a.Name and ID>a.ID)=0方法7:
select * from #T a where ID=(select top 1 ID from #T where Name=a.name order by ID desc)方法8:
select * from #T a where ID!<all(select ID from #T where Name=a.Name)方法9(注:ID为唯一时可用):
select * from #T a where ID in(select max(ID) from #T group by Name)--SQL2005:方法10:
select ID,Name,Memo from (select *,max(ID)over(partition by Name) as MinID from #T a)T where ID=MinID方法11:
select ID,Name,Memo from (select *,row_number()over(partition by Name order by ID desc) as MinID from #T a)T where MinID=1生成结果2:
/*
ID Name Memo
----------- ---- ----
3 A A3
5 B B2(2 行受影响)
*/--2、删除重复记录有大小关系时,保留大或小其中一个记录
--> --> (Roy)生成測試數據if not object_id('Tempdb..#T') is null
drop table #T
Go
Create table #T([ID] int,[Name] nvarchar(1),[Memo] nvarchar(2))
Insert #T
select 1,N'A',N'A1' union all
select 2,N'A',N'A2' union all
select 3,N'A',N'A3' union all
select 4,N'B',N'B1' union all
select 5,N'B',N'B2'
Go--I、Name相同ID最小的记录(推荐用1,2,3),保留最小一条
方法1:
delete a from #T a where exists(select 1 from #T where Name=a.Name and ID<a.ID)方法2:
delete a from #T a left join (select min(ID)ID,Name from #T group by Name) b on a.Name=b.Name and a.ID=b.ID where b.Id is null方法3:
delete a from #T a where ID not in (select min(ID) from #T where Name=a.Name)方法4(注:ID为唯一时可用):
delete a from #T a where ID not in(select min(ID)from #T group by Name)方法5:
delete a from #T a where (select count(1) from #T where Name=a.Name and ID<a.ID)>0方法6:
delete a from #T a where ID<>(select top 1 ID from #T where Name=a.name order by ID)方法7:
delete a from #T a where ID>any(select ID from #T where Name=a.Name)select * from #T生成结果:
/*
ID Name Memo
----------- ---- ----
1 A A1
4 B B1(2 行受影响)
*/
--II、Name相同ID保留最大的一条记录:方法1:
delete a from #T a where exists(select 1 from #T where Name=a.Name and ID>a.ID)方法2:
delete a from #T a left join (select max(ID)ID,Name from #T group by Name) b on a.Name=b.Name and a.ID=b.ID where b.Id is null方法3:
delete a from #T a where ID not in (select max(ID) from #T where Name=a.Name)方法4(注:ID为唯一时可用):
delete a from #T a where ID not in(select max(ID)from #T group by Name)方法5:
delete a from #T a where (select count(1) from #T where Name=a.Name and ID>a.ID)>0方法6:
delete a from #T a where ID<>(select top 1 ID from #T where Name=a.name order by ID desc)方法7:
delete a from #T a where ID<any(select ID from #T where Name=a.Name)
select * from #T
/*
ID Name Memo
----------- ---- ----
3 A A3
5 B B2(2 行受影响)
*/--3、删除重复记录没有大小关系时,处理重复值
--> --> (Roy)生成測試數據
if not object_id('Tempdb..#T') is null
drop table #T
Go
Create table #T([Num] int,[Name] nvarchar(1))
Insert #T
select 1,N'A' union all
select 1,N'A' union all
select 1,N'A' union all
select 2,N'B' union all
select 2,N'B'
Go方法1:
if object_id('Tempdb..#') is not null
drop table #
Select distinct * into # from #T--排除重复记录结果集生成临时表#truncate table #T--清空表insert #T select * from # --把临时表#插入到表#T中--查看结果
select * from #T/*
Num Name
----------- ----
1 A
2 B(2 行受影响)
*/--重新执行测试数据后用方法2
方法2:alter table #T add ID int identity--新增标识列
go
delete a from #T a where exists(select 1 from #T where Num=a.Num and Name=a.Name and ID>a.ID)--只保留一条记录
go
alter table #T drop column ID--删除标识列--查看结果
select * from #T/*
Num Name
----------- ----
1 A
2 B(2 行受影响)*/--重新执行测试数据后用方法3
方法3:
declare Roy_Cursor cursor local for
select count(1)-1,Num,Name from #T group by Num,Name having count(1)>1
declare @con int,@Num int,@Name nvarchar(1)
open Roy_Cursor
fetch next from Roy_Cursor into @con,@Num,@Name
while @@Fetch_status=0
begin
set rowcount @con;
delete #T where Num=@Num and Name=@Name
set rowcount 0;
fetch next from Roy_Cursor into @con,@Num,@Name
end
close Roy_Cursor
deallocate Roy_Cursor--查看结果
select * from #T
/*
Num Name
----------- ----
1 A
2 B(2 行受影响)
*/
1表示随便给的一个数字如:select 1 from tb
你用其他的也行.
数学1 起什么作用-->相当于*,没什么区别
所以那里可以填数字1 其实随便填一个非假的值都可以了
not exists 像 not in,
返回值为真的时候就显示,不然就不显示..
返回值为真的时候就显示,不然就不显示..exists 像 in,
返回值为真的时候就显示,不然就不显示..
where not exists(select 1 from tb where stuNo=t.stuNo and stuName=stuName and [no] < t.no)
语句有点问题吧?