solving a problem with cursors

问题

I have a question. I am working on cursors. Each time, after fetching the last records and printing its data’s, the cursor prints an addition line. To understand what I mean please consider the following sample example: I want to print the information about only 10 customers.

USE Northwind
GO

DECLARE myCursor CURSOR 
FOR SELECT TOP(10) ContactName FROM Customers
DECLARE @RowNo int,@ContactName nvarchar(30)
SET @RowNo=1
OPEN myCursor
FETCH NEXT FROM myCursor INTO @ContactName
PRINT  LEFT(CAST(@rowNo as varchar) + '      ',6)+'  '+ @ContactName
SET @RowNo=@RowNo+1
SET @ContactName=''
WHILE @@FETCH_STATUS=0
  BEGIN
        FETCH NEXT FROM myCursor INTO @ContactName
        PRINT + LEFT(CAST(@rowNo as varchar) + '      ',6)+'  '+ @ContactName
        SET @RowNo=@RowNo+1
        SET @ContactName=''
  END
CLOSE myCursor
DEALLOCATE myCursor

Now look at the output:

1       Maria Anders
2       Ana Trujillo
3       Antonio Moreno
4       Thomas Hardy
5       Christina Berglund
6       Hanna Moos
7       Frédérique Citeaux
8       Martín Sommer
9       Laurence Lebihan
10      Elizabeth Lincoln
11      

The row number 11 also has been printed. Is it a problem in a cursor or it always occurs? Is there any way not to print this addition data? Thanks (i use sql erver 2008)

回答1:

Either...

FETCH NEXT FROM myCursor INTO @ContactName
WHILE @@FETCH_STATUS = 0
BEGIN
    -- do stuff

    FETCH NEXT FROM myCursor INTO @ContactName
END

Or...

WHILE @@FETCH_STATUS = 0
BEGIN
    FETCH NEXT FROM myCursor INTO @ContactName
    IF @@FETCH_STATUS = 0
    BEGIN
        -- do stuff
    END
END

Or...

WHILE (1 = 1)
BEGIN
    FETCH NEXT FROM myCursor INTO @ContactName
    IF @@FETCH_STATUS <> 0
        BREAK

    -- do stuff
END

回答2:

You mentioned you're using SQL Server 2008. With SQL Server 2005 or greater, you don't need a cursor at all to do what you want.

select top 10 left(cast(row_number() over(order by ContactName) as varchar)+ '      ', 6) + ContactName
    from Customers

回答3:

See how you have the printing logic duplicated? That's a pointer to what's going wrong. Your loop should look like this:

FETCH NEXT INTO @working_variables
WHILE @@FETCH_STATUS = 0
    -- process @working_variables
    FETCH NEXT INTO @working_variables

The only duplicated code should be the FETCH NEXT itself - the way you have it now, the last FETCH happens, but you PRINT a line before the WHILE can exit.

回答4:

A FETCH at the end of the record set sets @@FETCH_STATUS to not 0.

The FETCH NEXT command should be the last line in the WHILE BLOCK.

USE Northwind
GO

DECLARE myCursor CURSOR 
FOR SELECT TOP(10) ContactName FROM Customers
DECLARE @RowNo int,@ContactName nvarchar(30)
SET @RowNo=0
OPEN myCursor
FETCH NEXT FROM myCursor INTO @ContactName
WHILE @@FETCH_STATUS=0
  BEGIN

        SET @RowNo=@RowNo+1
        SET @ContactName=''
        PRINT + LEFT(CAST(@rowNo as varchar) + '      ',6)+'  '+ @ContactName
        FETCH NEXT FROM myCursor INTO @ContactName
  END
CLOSE myCursor
DEALLOCATE myCursor

回答5:

This is an off-by-one error. Here's a better way to iterate through a cursor, w/ less code duplication:

USE Northwind
GO

DECLARE myCursor CURSOR 
FOR SELECT TOP(10) ContactName FROM Customers
DECLARE @RowNo int,@ContactName nvarchar(30)
SET @RowNo=0 -- initialize counters at zero, increment after the fetch/break
OPEN myCursor
WHILE 1=1 BEGIN -- start an infinite loop
  FETCH NEXT FROM myCursor INTO @ContactName
  IF @@FETCH_STATUS <> 0 BREAK
  SET @RowNo=@RowNo+1
  PRINT  LEFT(CAST(@rowNo as varchar) + '      ',6)+'  '+ @ContactName
END
CLOSE myCursor
DEALLOCATE myCursor

For extra points, use a cursor variable and declare w/ FAST_FORWARD and TYPE_WARNING, or STATIC for small datasets. eg:

DECLARE @cursor CURSOR
SET @cursor = CURSOR FAST_FORWARD TYPE_WARNING FOR
  SELECT TOP (10) ContactName FROM Customers
OPEN @cursor 
......
CLOSE @cursor
DEALLOCATE @cursor

CLOSE and DEALLOCATE are not strictly necessary, as the cursor variable will go out of scope at the end of the batch. It is still good form, however, as you might add more code at the end later on, and you should free up resources as early as possible.

TYPE_WARNING tells you when SQL Server implicitly converts the requested cursor type (FAST_FORWARD) to another type (typically STATIC), if the requested type is incompatible w/ your SELECT statement.

来源：https://stackoverflow.com/questions/3608916/solving-a-problem-with-cursors