Joins are used to define the relationship between Data Foundation objects. A join will help us combine a table's rows using one or multiple shared columns. A shared column will be the primary key for one table and foreign key in another table. In this chapter, we have already added many objects, such as physical database tables, alias tables, and derived tables, to
NorthWind Data Foundation. If we didn't define the relationship between these objects, the Universe integrity will not be satisfied. Indeed, the objects will not be linked, so the results will be inconsistent because of product (Cartesian) joins generated by the SQL queries.
A join will define only one relationship between two tables. For example, in
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