SQL WITH keyword inside CURSOR - sql

Question

Observing some strange behavior when using WITH keyword inside CURSOR:
WITH minuteList (aMinute) AS
(
SELECT #startTime UNION ALL
SELECT DATEADD(MINUTE, 1, aMinute)
FROM minuteList
WHERE aMinute < DATEADD(MINUTE, 9, #startTime)
)
SELECT * FROM minuteList
The above code works perfectly creating a table with one aMinute column and 10 datetime rows with 1 minute interval. However, the code below enters into an infinite loop printing only #startTime value endlessly.
DECLARE cursor1 CURSOR FOR
WITH minuteList (aMinute) AS
(
SELECT #startTime UNION ALL
SELECT DATEADD(MINUTE, 1, aMinute)
FROM minuteList
WHERE aMinute < DATEADD(MINUTE, 9, #startTime)
)
SELECT * FROM minuteList
OPEN cursor1
FETCH NEXT FROM cursor1 INTO #laterTime
WHILE ##FETCH_STATUS = 0 BEGIN
PRINT #laterTime
END
CLOSE cursor1;
DEALLOCATE cursor1;
Can someone explain what's happening and why the infinite loop occurs?

Answer 1

I have done a line-by-line analysis of your cursor call to explain why it goes into an infinite loop.
OPEN cursor1 // Opens your cursor
FETCH NEXT FROM cursor1 INTO #laterTime // Takes the first row returned by the
cursor and inserts it into the
variable #laterTime
WHILE ##FETCH_STATUS = 0 BEGIN // Checks if the next value has been
fetched and executes
the code inside the loop
PRINT #laterTime // Prints the current row
END // Ends the loop when FETCH_STATUS is
not 0 (when fetch fails).
CLOSE cursor1; // Closes the cursor.
The cause for the infinite loop is the fact that ##FETCH_STATUS is never changed because you have accessed only the first row from the cursor (the fetch has not failed because it has not been called more than once). After printing #laterTime, you need to fetch the next value of the cursor inside the loop. And only when that fetch fails will the loop terminate.
Consider this example as a reference.
Modified code will look like this:
OPEN cursor1
FETCH NEXT FROM cursor1 INTO #laterTime
WHILE ##FETCH_STATUS = 0 BEGIN
PRINT #laterTime
FETCH NEXT FROM cursor1 INTO #laterTime
END
CLOSE cursor1;