SQL Table Design Issue

Question

So I am building out a set of tables in an existing database at the moment, and have run into a weird problem.

First things first, the tables in question are called Organizations , Applications , and PostOrganizationsApplicants .

Organizations is a pre-existing table that is already populated with lots of data in regards to an organization's information which has been filled out in another form on another portal. EDIT: I cannot edit this table.
Applications is a table that records all information that a user inputs in the application form of the website. It is a new table.
PostOrganizationsApplicants is basically a copy of Organizations. This is also a new table.

The process goes:
1. Go to website and choose between two different web forms, Form A pertains to companies who are in the Organizations table, and Form B pertains to companies who are not in that table.
2a. If Form A is chosen, a lot of the fields in the application will be auto-populated because of their previous submission.
2b. If Form B is chosen, the company has to start from scratch and fill out the entire application from scratch.
3. Any Form B applicants must go into the PostOrganizationsApplicants table.

Now I am extremely new to SQL and Database Management so I may sound pretty stupid, but when I am linking the Organizations and PostOrganizationsApplicants tables to the Applications table, FK's for the OrganizationsID column and PostOrganizationsApplicantsID columns will have lots of empty spaces.
Is this good practice? Is there a better way to structure my tables? I've been racking my brain over this and just can't figure out a better way.

Answer 1

No, it's not necessarily bad practice to allow NULL values for foreign key columns.

If an instance of an entity doesn't have a relationship to an instance of another entity, then storing a NULL in the foreign key column is the normative practice.

From your description of the use case, a "Form A" Applications won't be associated with a row in Organizations or a row in PostOrganizationsApplicants .

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Getting the cardinality right is what is important. How many Organizations can a given Applications be related to? Zero? One? More than One? And vice versa.

If the relationship is many-to-many, then the usual pattern is to introduce a third relationship table.

Less frequently, we will also implement a relationship table for very sparse relationships, when a relationship to another entity is an exception, rather than the rule.

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I'm assuming that the OrganizationsID column you are referring to is in the PostOrganizationsApplicants table (which would mean that a PostOrganizationsApplicants can be associated with (at most) one Organizations .

I'm also assuming that PostOrganizationsApplicantsID column is in the Applications table, which means an instance of Applications can be associated with at most one PostOrganizationsApplicants .

Bottomline, have a "zero-or-one to many" relationship is valid, as long as that supports a suitable representation of the business model.

Answer 2

Why not just add a column to the Organizations table that indicates that the Organization is a "Post" type of organization and set it for the Form B type of applicants? - then, all your orgs are in one table - with a single property to indicate where they came from.

Answer 3

If you can add a new record to Organizations (I hope you can) just create FK from Organizations as PK of PostOrganizationsApplicants . So if Organizations has corresponding record in PostOrganizationsApplicants - it's "Post"!

Answer 4

Thanks everybody, I think I found the most efficient way to do it inspired by all of your answers.

My solution below, in case anyone else has a similar problem...

Firstly I will make the PK of PostOrganizationsApplicants the FK of Organizations by making a "link" table. Then I am going to add a column in PostOrganizationsApplicants which will take in a true/false value on whether they completed the form from the other portal or not. Then I will ask a question in the form whether they have already done the other version of the form or not. If the boolean value is true, then I will point those rows to the Organizations table to auto-populate the forms.

Thanks again!