Microsoft® SQL Server® 2012 Bible

157

Chapter 7: Relational Database Design and Creating the Physical Database Schema

7

FIGURE 7-6

The many-to-many logical model shows multiple tuples on both ends of the relationship.

Product Special Offer

Many-to-many relationships are nearly always optional. For example, the many products-

to-many special offers relationship is optional because the product and the special offer are

each valid without the other.

The one-to-one and the one-to-many relationship can typically be constructed from items

within an organization that users can describe and understand. That’s not always the case

with many-to-many relationships.

To implement a many-to-many relationship in SQL DDL, a third table, called an associative

table (sometimes called a junction table) is used, which artifi cially creates two one-to-many

relationships between the two entities (see Figure 7-7).

FIGURE 7-7

The many-to-many implementation adds an associative table to create artifi cial one-to-many

relationships for both tables.

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