更新时间:2023-09-10 16:23:46
首先,您应该绝对确保需要遍历每一行-基于集合的操作在我能想到的每种情况下都会执行得更快,并且通常会使用简单的代码.
First of all you should be absolutely sure you need to iterate through each row — set based operations will perform faster in every case I can think of and will normally use simpler code.
取决于您的数据,可能仅使用SELECT
语句即可进行循环,如下所示:
Depending on your data it may be possible to loop using just SELECT
statements as shown below:
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
另一种替代方法是使用临时表:
Another alternative is to use a temporary table:
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数量.
The option you should choose really depends on the structure and volume of your data.
注意::如果您使用的是SQL Server,则***使用:
Note: If you are using SQL Server you would be better served using:
WHILE EXISTS(SELECT * FROM #Temp)
使用COUNT
必须触摸表中的每一行,EXISTS
只需要触摸第一行(请参见下面的约瑟夫的答案.
Using COUNT
will have to touch every single row in the table, the EXISTS
only needs to touch the first one (see Josef's answer below).