Database Reference
In-Depth Information
Figure 2-1.
A one-to-many relationship
Many-to-many (M:M)
: For each row in Table A, there are zero or more related
rows in Table B, and vice versa. Many-to-many relationships are not so easy to
achieve, and they require a special technique to implement them. This
relationship is actually implemented in a one-many-one format, so it requires a
third table (often referred to as a
junction table)
to be introduced in between
that serves as the path between the related tables.
This is a very common relationship. Figure 2-2 shows an example from Northwind: an order can
have many products, and a product can belong to many orders. The Order Details table not only
represents the M:M relationship but also contains data about each particular order-product
combination.
