Parent/Child Tables Query Pattern - sql

Question

Suppose I have the following parent/child table relationship in my database:
TABLE offer_master( offer_id int primary key, ..., scope varchar )
TABLE offer_detail( offer_detail_id int primary key, offer_id int foreign key, customer_id int, ... )
where offer_master.scope can take on the value
INDIVIDUAL: when the offer is to made to particular customers. In this case,
whenever a row is inserted into offer_master, a corresponding row is
added to offer_detail for each customer to which the offer has been extended.
e.g.
INSERT INTO offer_master( 1, ..., 'INDIVIDUAL' );
INSERT INTO offer_detail( offer_detail_id, offer_id, customer_id, ... )
VALUES ( 1, 1, 100, ... )
INSERT INTO offer_detail( offer_detail_id, offer_id, customer_id, ... )
VALUES ( 2, 1, 101, ... )
GLOBAL: when the offer is made to all customers. In this case,
new offers can be added to the parent table as follows:
INSERT INTO offer_master( 2, ..., 'GLOBAL' );
INSERT INTO offer_master( 3, ..., 'GLOBAL' );
but a child row is added to offer_detail only
when a customer indicates some interest in the offer. So
it may be the case that, at some later point we will have
INSERT INTO offer_detail( offer_detail_id, offer_id, customer_id, ... )
VALUES ( 4, 3, 100, ... )
Given this situation, suppose we would like to query the database
to obtain all offers which have been extended to customer 100;
this includes 3 types of offers:
offers which have been extended specifically to customer 100.
global offers which customer 100 showed no interest in.
global offers which customer 100 did show interest in.
I see two approaches:
Using a Subquery:
SELECT *
FROM offer_master
WHERE offer_id in (
SELECT offer_id
FROM offer_detail
WHERE customer_id = 100 )
OR scope = 'GLOBAL'
Using a UNION
SELECT om.*
FROM offer_master om INNER JOIN
offer_detail od
ON om.offer_id = od.offer_id
WHERE od.customer_id = 100
UNION
SELECT *
FROM offer_master
WHERE scope = 'GLOBAL'
Note: a UNION ALL cannot be used since a global offer
which a customer has shown interest in would be duplicated.
My question is:
Does this query pattern have a name?
Which of the two query methods are preferable?
Should the database design be improved in some way?

Answer 1

I'm not aware of a pattern name.
To me, the second query is clearer but I think either is OK.
offer_detail seems to be a dual purpose table which is a bit of a red flag to me. You might have separate tables for the customers in an individual offer, and the customers who have expressed interest.