简单说明一下，如果你使用的是SQL Server(2008及以上版本)，下面的例子有:

While (Select Count(*) From #Temp) > 0

会不会更好

While EXISTS(SELECT * From #Temp)

Count将不得不触及表中的每一行，EXISTS只需要触及第一行。

这是我使用的2008 R2代码。我使用的这段代码是在所有故事的关键字段(SSNO & EMPR_NO)上构建索引

if object_ID('tempdb..#a')is not NULL drop table #a

select 'IF EXISTS (SELECT name FROM sysindexes WHERE name ='+CHAR(39)+''+'IDX_'+COLUMN_NAME+'_'+SUBSTRING(table_name,5,len(table_name)-3)+char(39)+')' 
+' begin DROP INDEX [IDX_'+COLUMN_NAME+'_'+SUBSTRING(table_name,5,len(table_name)-3)+'] ON '+table_schema+'.'+table_name+' END Create index IDX_'+COLUMN_NAME+'_'+SUBSTRING(table_name,5,len(table_name)-3)+ ' on '+ table_schema+'.'+table_name+' ('+COLUMN_NAME+') '   'Field'
,ROW_NUMBER() over (order by table_NAMe) as  'ROWNMBR'
into #a
from INFORMATION_SCHEMA.COLUMNS
where (COLUMN_NAME like '%_SSNO_%' or COLUMN_NAME like'%_EMPR_NO_')
    and TABLE_SCHEMA='dbo'

declare @loopcntr int
declare @ROW int
declare @String nvarchar(1000)
set @loopcntr=(select count(*)  from #a)
set @ROW=1  

while (@ROW <= @loopcntr)
    begin
        select top 1 @String=a.Field 
        from #A a
        where a.ROWNMBR = @ROW
        execute sp_executesql @String
        set @ROW = @ROW + 1
    end 

Step1:下面的select语句为每条记录创建一个具有唯一行号的临时表。

select eno,ename,eaddress,mobno int,row_number() over(order by eno desc) as rno into #tmp_sri from emp 

步骤2:声明所需的变量

DECLARE @ROWNUMBER INT
DECLARE @ename varchar(100)

步骤3:从临时表中获取总行数

SELECT @ROWNUMBER = COUNT(*) FROM #tmp_sri
declare @rno int

Step4:根据在temp中创建的唯一行号循环temp表

while @rownumber>0
begin
  set @rno=@rownumber
  select @ename=ename from #tmp_sri where rno=@rno  **// You can take columns data from here as many as you want**
  set @rownumber=@rownumber-1
  print @ename **// instead of printing, you can write insert, update, delete statements**
end

这将在SQL SERVER 2012版本中工作。

declare @Rowcount int 
select @Rowcount=count(*) from AddressTable;

while( @Rowcount>0)
  begin 
 select @Rowcount=@Rowcount-1;
 SELECT * FROM AddressTable order by AddressId desc OFFSET @Rowcount ROWS FETCH NEXT 1 ROWS ONLY;
end 

以下是我的做法:

Select Identity(int, 1,1) AS PK, DatabaseID
Into   #T
From   @databases

Declare @maxPK int;Select @maxPK = MAX(PK) From #T
Declare @pk int;Set @pk = 1

While @pk <= @maxPK
Begin

    -- Get one record
    Select DatabaseID, Name, Server
    From @databases
    Where DatabaseID = (Select DatabaseID From #T Where PK = @pk)

    --Do some processing here
    -- 

    Select @pk = @pk + 1
End

[编辑]因为我可能在第一次读这个问题时跳过了“变量”这个词，这里是一个更新的回答…

declare @databases table
(
    PK            int IDENTITY(1,1), 
    DatabaseID    int,
    Name        varchar(15),   
    Server      varchar(15)
)
-- insert a bunch rows into @databases
--/*
INSERT INTO @databases (DatabaseID, Name, Server) SELECT 1,'MainDB', 'MyServer'
INSERT INTO @databases (DatabaseID, Name, Server) SELECT 1,'MyDB',   'MyServer2'
--*/

Declare @maxPK int;Select @maxPK = MAX(PK) From @databases
Declare @pk int;Set @pk = 1

While @pk <= @maxPK
Begin

    /* Get one record (you can read the values into some variables) */
    Select DatabaseID, Name, Server
    From @databases
    Where PK = @pk

    /* Do some processing here */
    /* ... */ 

    Select @pk = @pk + 1
End

首先，你应该绝对确定你需要遍历每一行——基于行集的操作在我能想到的任何情况下都会执行得更快，通常会使用更简单的代码。

根据您的数据，可能只使用SELECT语句循环，如下所示:

Declare @Id int

While (Select Count(*) From ATable Where Processed = 0) > 0
Begin
    Select Top 1 @Id = Id From ATable Where Processed = 0

    --Do some processing here

    Update ATable Set Processed = 1 Where Id = @Id 

End

另一种方法是使用临时表:

Select *
Into   #Temp
From   ATable

Declare @Id int

While (Select Count(*) From #Temp) > 0
Begin

    Select Top 1 @Id = Id From #Temp

    --Do some processing here

    Delete #Temp Where Id = @Id

End

您应该选择的选项实际上取决于数据的结构和容量。

注意:如果您正在使用SQL Server，您将更好地使用:

WHILE EXISTS(SELECT * FROM #Temp)

使用COUNT将不得不触及表中的每一行，EXISTS只需要触及第一行(参见Josef的回答)。