Explain how to link tables in (1:1) association. Why may this be different in a (1:0,1) association?

Answer 1

Where a true 1:1 association exists between tables, either (or both) primary keys may be embedded as foreign keys in the related table. On the other hand, when the lower cardinality value is zero (1:0,1) a more efficient table structure can be achieved by placing the one-side (1:) table's primary key in the zero-or-one (:0,1) table as a foreign key. Assume that a company has 1000 employees but only 100 of them are sales staff. Assume also that each sales person is assigned a company car. Therefore, every occurrence in the Employee entity is associated with either zero or one occurrence in the Company Car entity. If we assigned the Company Car (:0,1) side primary to the Employee (:1) table as a foreign key then most of the foreign will have null (blank) values. While this approach would work, it could cause some technical problems during table searches. Correctly applying the key-assignment rule solves this problem because all Company Car records will have an employee assigned and no null values will occur.