Re: MERGE Specification

On 06/08/10 04:39, Boxuan Zhai wrote:
> I have seen a lively discussion about the DO NOTING action in MERGE command.
> And, I think most people want it. So it will be added to my next patch.
>
> Before the implementation, I still have some questions to confirm:
>
> 1. If we have a DO NOTHING action specified, it should be the last WHEN
> clause. It must be of the NOT MATCHED cases, and it CANNOT have any
> additional action qualifications. Am I correct?

It would be useful to specify it in WHEN MATCHED sometimes, and not 
necessarily the last. For example:

MERGE INTO Stock S
USING DailySales DS ON S.Item = DS.Item
WHEN MATCHED AND (QtyOnHand ‐ QtySold = 0) THEN DELETE
WHEN MATCHED THEN UPDATE SET QtyOnHand = QtyOnHand ‐ QtySold -- Don't add new inactive items to stock if not there
already
WHEN MATCHED AND (itemtype = 'inactive') THEN DO NOTHING
WHEN NOT MATCHED THEN INSERT VALUES (Item, QtySold);

It shouldn't be difficult to support DO NOTHING in all cases, right?

> 2. If no DO NOTHING specified, we will imply a INSERT DEFAULT VALUES action
> as the end of MERGE.
> My question is, is this action taken only for the NOT MATCHED tuples?  If
> this is the case, then what about the MATCHED tuples that match not previous
> actions? Ignore them?
> That means we are in fact going to add two implicit WHEN clause:
>                 a) WHEN NOT MATCHED INSERT default values;
>                 b) WHEN MATCHED THEN DO NOTHING.
> OR, is the INSERT DEFAULT VALUES applied to ALL tuples not matter they are
> MATCHED or not?

We'll need to figure out what the SQL standard says about this. I tried 
reading the spec but couldn't readily understand what the default action 
should be. Does someone else know that? What do other DBMSs do?

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com