In simple english,

Main Query:

delete any record from the child where the sub query returns at least
one record.

SubQuery:

Return the value 1 every time you come across a record in the parent
with a child ID equal to the current child we are checking, and the
parent f_foo equals 1.

Helen will have to tell you if the following query would provide any
benefit:

DELETE FROM Child c
WHERE EXISTS(
SELECT FIRST 1 1 FROM Parent p
WHERE p.F_CHILD_ID = c.F_ID AND p.F_FOO = 1)

I have added the 'first 1' directive to say only return the first
record. It is logically equivalent but just might save firebird some work.

In either case, I am sure you have a reason not to, but isn't it
standard to define a foreign key if you are going to join on that
record, or if that is not possible, check that your tables are
normalised. I am also not sure why you don't want to use the Before
Delete Trigger to say something like:

delete from parent
where f_child_id = old.f_id
and f_foo = 1

Also, make sure you have at least have an index (except if you have a
foreign key constraint which will add one for you).



--- In firebird-support@yahoogroups.com, "robert_difalco"
<rdifalco@t...> wrote:

>
> --- In firebird-support@yahoogroups.com, Helen Borrie <helebor@t...>
> wrote:
> >
> > DELETE FROM Child c
> > WHERE EXISTS(
> > SELECT 1 FROM Parent p
> > WHERE p.F_CHILD_ID = c.F_ID AND p.F_FOO = 1)
> >
>
> Helen, could you explain how this query works? Is it standard SQL that
> can be used across different database server?
>
> I don't think understand exactly how this is parsed and how the
> relation between Parent and Child is made here. The JOIN I understand,
> even the Sub-Select I get, but I don't grok this one.
>
> TIA,
>
> R.