PL/SQL Merge statement - sql

Question

I'm looking to implement a sort of Type 2 Slow Changing Dimension like behavior into my dimension table using PL/SQL's MERGE statement. It workd just fine for updating existing values and inserting new ones. I'm looking to extend this functionality by not only updating existing values but creating a different row with the updated values while preserving the row with the "outdated" values.
In short, is it possible to do this?
MERGE INTO A
USING B
ON (A.ID = B.ID)
WHEN MATCHED THEN
UPDATE END_DATE ON THE EXISTING ROW;
INSERT UPDATED VALUES IN A NEW ROW;
WHEN NOT MATCHED THEN
INSERT A NEW ROW WITH NEW VALUES;
Thanks you guys in advance.

Answer 1

If you are truly doing a Type 2 dimension and want to use a merge, yes it can be done but it's not terribly straightforward. Effectively, if you will need to compare your data using an inline view in the USING clause, that features a column that indicates whether its an insert or an update. This field gets joined to an slowly changing dimension table that drives whether an insert or an update occurs.
This blog post describes the technique in great detail and has worked for us, albeit we used a hash to determine unqiueness as opposed to a column by column compare.
Load Slowly Changing Dimension Type 2 using Oracle Merge Statement

Answer 2

You need to run two statements instead of 1 here.
(in your pseudo-code)
UPDATE END_DATE IN A WHERE A.ID = something_from_B
INSERT new VALUES IN A
The insert needs to happen no matter what. So no need of checking if the row exists or not (and only updating if the row exists). Just end_date all records that match B. This would end_date 0 to n number of rows. And then Insert everything together without worrying if that record has been end-dated already or not.
Put this inside a BEGIN...END; if you want transactional atomicity.

Answer 3

In short - no. A MERGE statement provides the option to INSERT if matching data is not found, and to UPDATE if matching data is found. It does NOT give the option to "update-and-insert" if matching data is found.
Best of luck.

Answer 4

As I understand, you wrote a pseudocode. So I can suggest just an idea also in pseudocode:
MERGE INTO A
USING (select * from B1 union all
select * from B2) B
ON (A.ID = B.ID)
WHEN MATCHED THEN
UPDATE END_DATE ON THE EXISTING ROW FROM B1;
WHEN NOT MATCHED THEN
INSERT A NEW ROW WITH NEW VALUES FROM B2;
If you want to insert updated rows as new, you can generate subquery based on B, which will contain "updated rows", but these rows have to be considered as new by SQL engine (it is impossible to say how to do it without details about your tables). In my query B1 is "rows to update", and B2 is "updated rows which have to look as new". If you do this, updated rows will be inserted.
Also, no guarantee that in your case it could be implemented.