How the tables in a relational database are related

Concepts

• The tables in a relational database are related to each other through their key columns. For example, the categoryID column is used to relate the categories and products tables above.
• The categoryID column in the products table is called a foreign key because it identifies a related row in the categories table. A table may contain one or more foreign keys.
• When you define a foreign key, you can’t add rows to the table with the foreign key unless there’s a matching primary key in the related table.
• The relationships between the tables in a database correspond to the relationships between the entities they represent. The most common type of relationship is a one-to-many relationship as illustrated by the categories and products table.
• A table can also have a one-to-one relationship or a many-to-many relationship with another table.