Practical SQL Handbook, The: Using SQL Variants, Fourth Edition

Why Joins Are Necessary

In a database that has been designed according to the normalization rules, one table might not give you all the information you need about a particular entity. For comprehensive data analysis, you must assemble data from several tables. The relational model—having led you to partition your data into several single-subject tables following the rules of normalization and good, clean database design—relies on the join operation to enable you to perform ad hoc queries and produce comprehensible reports. In the same way, you are not limited in the kind of new tables you can add to the database. Thus the join operation is one of the key operations of the relational model. Joins are possible because of the relational model, and ...