Transform Logical Data model to SQL Table design

Question

In a LDM I recently made, I have an entity which has the following structure:

Building_ID (Primary Key, Foreign Key),
Plant_ID (Foreign Key),
Build_Year (Primary Key),
Size

I need to create a table in a SQL database using this design. The question I'm running into is how do I handle the primary keys here? Is it OK for a SQL table to have multiple primary keys? If the answer to this question is yes, then which column should act as the unique index? Should I create a new column to act as the unique index identifier?

Answer 1

Any SQL table for any relational database system (SQL Server, Oracle, Firebird, IBM DB2, Sybase etc.) I know can only ever have one primary key - after all, it's the primary key - there can only ever be one.

However, a primary key can be made up from multiple columns (called a "compound primary key"). There are downsides such as: all foreign key constraints from other tables also must specify all columns in the compound PK, thus making joining the tables a bit of a pain (since you need to specify all equality constraints for all columns included in the key in your JOIN).

Besides a primary key, you can also have multiple alternate keys - other column(s) that could also identify the row uniquely. Those make excellent candidates for eg indices, if those can help you speed up access to the table (but don't over-index your tables! Less is more)