Question

Is there a best practice pattern to model the following parent child relationship where exactly one child can be flagged as the default child.

I can think of two ways. Which is better or is there another better alternate?

   - ParentTable
      - Id

   - ChildTable
      - Id
      - ParentId
      - IsDefault

or

   - ParentTable
      - Id
      - DefaultChildId

   - ChildTable
      - Id
      - ParentId

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Answer 1

I favor data models that prevent invalid data from existing at all. If you use the second approach and create a foreign key relationship between DefaultChildId and the ChildTable, then you can add constraints to ensure that your database can’t ever be in an invalid state — if you try to, say, delete a child that is somebody’s default child, the database will prevent you. Using the first approach, it won’t.

Specifically (as noted in a comment), ON DELETE RESTRICT constraint should be used on DefaultChildId and ON DELETE CASCADE on ParentID. This allows creation of records with more flexibility (you don’t need to have a default child already there to be able to make a parent), prevents accidental deletion of default child without first updating to a new default, and intelligently handles deletion of the parent.