Question

DB gurus,

I am hoping someone can set set me on the right direction.

I have two tables. Table A and Table B. When the system comes up, all entries from Table A are massaged and copied over to Table B (according to Table B’s schema). Table A can have tens of thousands of rows.

While the system is up, Table B is kept in sync with Table A via DB change notifications.

If the system is rebooted, or my service restarted, I want to re-initialize Table B. However, I want to do this with the least possible DB updates. Specifically, I want to:

• add any rows that are in Table A, but not in Table B, and
• delete any rows that are not in Table A, but are in Table B
• any rows that are common to Table A and Table B should be left untouched

Now, I am not a “DB guy”, so I am wondering what is conventional way of doing this.

Answer 1

Use exists to keep processing to a minimum.

Something along these lines, modified so the joins are correct (also verify that I didn’t do something stupid and get TableA and TableB backwards from your description):

insert into TableB
    select 
        *
    from
        TableA a
    where
        not exists (select 1 from TableB b where b.ID = a.ID)

delete from 
    TableB b
where
    not exists (select 1 from TableA a where a.ID = b.ID)