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Editorial Team
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Asked: 2026-06-02T16:20:15+00:00

i have two tables, one ‘master’ and one ‘child’ table. Each table has a

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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

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1 Answer

  1. Editorial Team

    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.

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