Determining Whether a Column is a Primary or External Key
You can determine whether a specific column is a primary key or external key of the table by consulting the reference page of the table. A list of links to the reference pages of all the installation database tables is located under Database Tables. Each of the reference pages identifies columns that are a primary key or an external key.
Database columns that are external keys take the name of the primary key column with an added underscore character. For example, an external key to the File column of the File table is always named File_.