Selecting Data from Multiple Tables
Having the capability to select data from multiple tables is one of SQL's most powerful features. Without this capability, the entire relational database concept would not be feasible. Single-table queries are sometimes quite informative, but in the real world, the most practical queries are those whose data is acquired from multiple tables within the database.
As you witnessed in the hour on normalization, a relational database is broken up into smaller, more manageable tables for simplicity and the sake of overall management ease. As tables are divided into smaller tables, the related tables are created with common columns—primary keys. These keys are used to join related tables to one another.
A join
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