How to Engineer Software by Steve Tockey

Author:Steve Tockey
Language: eng
Format: epub
Published: 2019-10-08T00:00:00+00:00

The general rule is to place foreign keys so they point at a table that has a ‐to‐exactly‐one multiplicity. If multiplicity is ‐to‐zero‐or‐one, a foreign key can be used, but access needs to be protected for the null case. Many to many or biconditional (zero or anything to zero or anything) should use a correlation table.

17.5.3 Association Classes

When the semantic model has an association class, use a correlation table in design—as in the many‐to‐many case. The correlation table can be keyed by the concatenation of the keys of the linked tables. There will also be non‐key fields for attributes in the semantic model. Book order line is an association class in Order fulfillment's semantic model. As shown in Figure 17.4, it is implemented as a table keyed by bookOrderId and mediumID but also has quantity as a non‐key field.

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