Question

I have this loop, which updates @UpdateBlockSize number of records on each iteration:

WHILE 1=1
BEGIN

    UPDATE TOP (@UpdateBlockSize) table1
    SET field1 = nv.value
    FROM table1 i
        INNER JOIN #table2 nv ON nv.id = i.id   
    IF @@ROWCOUNT < @UpdateBlockSize BREAK
END

Question:
How can I ensure that each iteration works within a transaction so that if it fails, only that iteration is rolled back and the LOOP exits?

Answer 1

Any single update that fails is it’s own transaction (assuming no explicit outer transaction).

If I understand you correctly…

• you want to handle errors gracefully
• you want 10 individual transactions
• if loop number, say, 7 fails, rollback that loop only and exit the loop
• loops 1-6 stay committed

So, try this

WHILE 1=1
BEGIN
    BEGIN TRY
       UPDATE TOP (@UpdateBlockSize) table1
       SET field1 = nv.value
       FROM table1 i
           INNER JOIN #table2 nv ON nv.id = i.id;
       SET @rc = @@ROWCOUNT
    END TRY
    BEGIN CATCH
       SET @rc = 0;
    END CATCH
    IF @@ROWCOUNT < @UpdateBlockSize BREAK
END

If you want a single transaction for all loops, then don’t use a loop. The transaction log will contain all rollback information for all loops because it is a single transaction. Unless this is abbreviated code, a loop adds no value in this case…