CODEWITHSUNDEEP
sqlsql-serverforeign-keys
PedroC88
PedroC88

Reputation: 3829

Recommended structure for table that has FK for 3 other tables

I have a table that will contain information for 3 other tables. The design I have is that this table will have a column that will tell the objects's ID and another column will tell the objects's type (and thus the table that that row refers to).

Two questions:
a) Is that the best design or is there something else more widely accepted?
b) What is the recommend procedure to assure that IDs are valid for the given objects's type?

Upvotes: 1

Views: 68

Answers (3)

Caleb Hattingh
Caleb Hattingh

Reputation: 9225

The basics of DB schema design are easy, but more complicated situations can be really complicated to figure out what's best. There is a lot of personal subjectivity that can come into play here, and even performance can be a factor in denormalizing a design.

Disclaimer aside, my personal recommendation is to never use a column to store more than one kind of FK, i.e. a column for FKs should store FKs that point only to a single table. If you don't do this, you have to map the cascade of that column's data into multiple sub-select queries inside your code, and it can begin to get more messy than you expected. Your given "Problem No. 2, ensuring validity between type and FK" is just the beginning of a whole world of pain that will cascade throughout your source code.

Assuming you change the design to use one field per FK reference, I would also check whether each FK field in your main "information-holding table" will be fully valid for each record. If not, I would move out the FK columns that will only be applicable some of the time to a separate table.

Upvotes: 0

Tasio
Tasio

Reputation: 351

a) I'm supposing you have a relationship one to many between objects and object types. In a normal design you'd have a reference from the objecttype column in the objects table to the primary key of the object types table

b) I would enforce referential integrity in the relationship properties (this depends on the dbms you are using). It's also up to you to use cascading on updates and deletes. This way, an update or a delete of the primary key on object types table would be reflected on the objects one, updating its foreign key column (object type column) or deleting the registers that have that object type.

Upvotes: 0

Heinzi
Heinzi

Reputation: 172380

If I understood your question correctly, each row in your table links to exactly one of the three other tables.

Your approach (type field + one foreign key field) is a valid design, and it's useful if you want to create a general-purpose table that contains meta-information about your data (e.g. a list of records that should be retransmitted for replication).

Another approach, which might be more suitable for real application-level data, would be to have three columns, each being a foreign key to one of the three tables, and to add a constraint that requires exactly two of those fields to be null. The has the following advantages:

  • The three FKs do not need to have the same data type.
  • The JOIN syntax becomes more natural (not involving the type field).
  • You can add referential integrity constraints on those FK columns.
  • You don't need to ensure correctness of the type field -- in fact, you don't need the type field at all. The type is determined implicitly by the one FK column which is not null.

Upvotes: 3

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