使用游标时如何避免在 T-SQL 中重复 FETCH？

Question

In T-SQL, when iterating results from a cursor, it seems to be common practice to repeat the FETCH statement before the WHILE loop. The below example from Microsoft :

DECLARE Employee_Cursor CURSOR FOR
SELECT EmployeeID, Title FROM AdventureWorks2012.HumanResources.Employee
    WHERE JobTitle = 'Marketing Specialist';
OPEN Employee_Cursor;

FETCH NEXT FROM Employee_Cursor;
WHILE @@FETCH_STATUS = 0
    BEGIN
        FETCH NEXT FROM Employee_Cursor;
    END;
CLOSE Employee_Cursor;
DEALLOCATE Employee_Cursor;
GO

(Notice how FETCH NEXT FROM Employee_Cursor; appears twice.)

If the FETCH selects into a long list of variables, then we have a large duplicated statement which is both ugly and of course, "non-DRY" code.

I'm not aware of a post-condition control-of-flow T-SQL statement so it seems I'd have to resort to a WHILE(TRUE) and then BREAK when @@FETCH_STATUS is not zero. This feels clunky to me.

What other options do I have?

Answer 1

There's a good structure posted online by Chris Oldwood which does it quite elegantly:

DECLARE @done bit = 0 

WHILE (@done = 0)  
BEGIN 
  -- Get the next author.  
  FETCH NEXT FROM authors_cursor  
  INTO @au_id, @au_fname, @au_lname  

  IF (@@FETCH_STATUS <> 0) 
  BEGIN 
    SET @done = 1 
    CONTINUE 
  END 

  -- 
  -- stuff done here with inner cursor elided 
  -- 
END

Answer 2

This is what I've resorted to (oh the shame of it):

WHILE (1=1)
BEGIN
    FETCH NEXT FROM C1 INTO
   @foo,
   @bar,
   @bufar,
   @fubar,
   @bah,
   @fu,
   @foobar,
   @another,
   @column,
   @in,
   @the,
   @long,
   @list,
   @of,
   @variables,
   @used,
   @to,
   @retrieve,
   @all,
   @values,
   @for,
   @conversion

    IF (@@FETCH_STATUS <> 0)
    BEGIN
        BREAK
    END

     -- Use the variables here
END

CLOSE C1
DEALLOCATE C1

You can see why I posted a question. I don't like how the control of flow is hidden in an if statement when it should be in the while .

Answer 3

The first Fetch shouldn't be a Fetch next , just a fetch .

Then you're not repeating yourself.

I'd spend more effort getting rid of the cursor, and less on DRY dogma, (but if it really matters, you could use a GOTO :) - Sorry, M. Dijkstra)

GOTO Dry
WHILE @@FETCH_STATUS = 0 
BEGIN 
    --- stuff here

Dry:
    FETCH NEXT FROM Employee_Cursor; 
END; 

Answer 4

Here is my humble contribution. Single FETCH statement, no GOTO, no BREAK, no CONTINUE.

-- Sample table
DROP TABLE IF EXISTS #tblEmployee;
CREATE TABLE #tblEmployee(ID int, Title varchar(100));
INSERT INTO #tblEmployee VALUES (1, 'First One'), (2, 'Second Two'), (3, 'Third Three'), (3, '4th Four');
-- Cursor with one FETCH statement
DECLARE @bEOF bit=0, @sTitle varchar(200), @nID int;
DECLARE cur CURSOR LOCAL FOR SELECT ID, Title FROM #tblEmployee;
OPEN cur;
WHILE @bEOF=0
BEGIN
    FETCH NEXT FROM cur INTO @nID, @sTitle;
    IF @@FETCH_STATUS<>0
        SET @bEOF=1;
    ELSE
    BEGIN
        PRINT Str(@nID)+'. '+@sTitle;
    END;
END;
CLOSE cur;
DEALLOCATE cur;
-- Cleanup
DROP TABLE IF EXISTS #tblEmployee;

Answer 5

It is obvious that a cursor is the pointer to the current row in the recordset. But mere pointing isn't gonna make sense unless it can be used. Here comes the Fetch statement into the scene. This takes data from the recordset, stores it in the variable(s) provided. so if you remove the first fetch statement the while loop won't work as there is not "FETCHED" record for manipulation, if you remove the last fetch statement, the "while" will not loop-through.

So it is necessary to have both the fetch statement to loop-through the complete recordset.

Answer 6

Simply said you can't... that's just how most where statements in SQL work. You need to get the first line before the loop and then do it again in the while statement.

The better question how to get rid of the cursor and try to solve your query without it.