Data is often part of a bigger picture with many interrelated data sources, as seen in relational databases. In such a world, queries often target multiple data sources simultaneously, by joining those on a certain criterion. In relational database technologies, this is usually realized by giving data records a unique identifier, which is referred to as a primary key. To refer to a data record from another table, a column is designated as a foreign key referring to the related table’s primary key.
Figure 19.31 shows such a relationship, sometimes also referred to as an association, in the context of LINQ to SQL. In this particular case, one category can have many products associated with it, hence the