Primary key on 2 tables

Question

i have two tables, one 'master' and one 'child' table. Each table has a field named 'ProductNo', which is defined as PRIMARY KEY and UNIQUE. Is it possible to define the field 'ProductNo' in the table 'child' and the same field in table 'master' as PRIMARY + UNIQUE together?

master:
ID | ProductNo

child:
ID | MasterID (FK on master.ID) | ProductNo

Relation >> 1 (master) : n (child) 


example data:
master: 
1 | 1234
2 | 4567

child:
100 | 1 | 3333
101 | 1 | 4444
102 | 2 | 5555
103 | 1 | 1234 <----- NOT ALLOWED! PRODUCT NO ALREADY EXISTING IN TABLE `MASTER`
104 | 2 | 1234 <----- NOT ALLOWED! PRODUCT NO ALREADY EXISTING IN TABLE `MASTER`

It is needed to check on inserting/updating table 'child' if 'ProductNo' already exists in table 'master'.

How can I define it? Or am I needed to create a trigger for this?

TIA Matt

Answer 1

no, there is no such thing as composite PKs among tables.

Just for data consistency, if the Ids are the same, you should add a FK from child to the master.

To solve your problem, a trigger with a check like this:

if exists (select 1 from master where prodcutId=new_productId)

would be a good idea

EDIT:

actually the best idea is to have only one table called product with a ID and a masterID field with a relation to itself. The way you have today Im pretty sure that you have a lot of duplicate data and you are stuck with 2 levels on hierarchy.

Answer 2

(Original answer) You can declare a foreign key from master to child, even if that foreign key points to the primary key of child. This would be a one to zero-or-one relationship, and is not that uncommon. A row cannot exist in child without a matching row in master already being inserted, but a row can exist in master without a matching child row. Your inserts therefore need to happen in the order master then child.

(Edited in light of question edit) HOWEVER, in your case, the column you are referring to looks like it should not actually be the primary key of either table, but rather you have a separate primary/foreign key, and the column in question needs to be unique across the two tables, which has become clear now you've edited some sample data into your question. In this case, you'd be best to use a trigger on both tables, to check existence in the other table and prevent the insert/update if the ProductNo already exists.

Answer 3

Just as @DavidM said, it can be done, but it seems you are with some modelling issues. First, if you have a natural primary key ProductNo, why do you define a surrogate ID? The other thing you might consider is to combine these two tables into a single one (as might make sense for most of 1-to-1 cases).

Answer 4

Are you sure you need the two tables? Keep just one, having productID plus parentID. Then productID can be a primary key and auto increment, while everything having a parentID other than null (f.keyed to the same table) would be a child item.

Answer 5

You can add a column named ProductNo in child table and add a foreign key reference to the parent table.