The join operation is the hallmark of the relational model of database management. More than any other feature, the join distinguishes relational database management systems from other types of database management systems.
In structured database management systems, often known as network and hierarchical systems, relationships between data values are predefined. Once a database has been set up, it is difficult to make queries about unanticipated relationships among the data.
In a relational database management system, relationships among data values are left unstated in the definition of a database. They become explicit when the data is manipulated—when you query the database, not when you create it. You can ask any question that comes to mind about the data stored in the database, regardless of what was intended when the database was set up.
According to the rules of good database design, called normalization rules, each table should describe one kind of entity—a person, place, event, or thing. That is why, when you want to compare information about two or more kinds of entities, you need the join operation. Relationships among data stored in different tables are discovered by joining them.
A corollary of this rule is that the join operation gives you unlimited flexibility in adding new kinds of data to your database. You can always create a new table that contains data about a different kind of entity. If the new table has a field with values similar to those in some field of an existing table or tables, it can be linked to those other tables by joining.