Primary and Foreign Keys

A relational database is a type of database that stores data in the form of tables with columns and rows, and uses relationships between tables to organize data in a meaningful way.

Primary keys and foreign keys are two important concepts in relational databases that help maintain the integrity and relationships of data stored in the tables.

A primary key is a unique identifier for each record (or row) in a table. It is used to uniquely identify a record in a table and enforce the relational integrity of data. A primary key must contain unique values for every record in a table, and it cannot contain null values.

A foreign key is a field in a table that is a primary key in another table. It is used to establish and enforce a relationship between two tables. In a relational database, a foreign key is used to link data in one table to corresponding data in another table. A foreign key can be thought of as a way to enforce referential integrity, meaning that the value in the foreign key must match a value in the referenced table's primary key.

For example, consider two tables in a relational database: a "customers" table and an "orders" table. The "customers" table has a primary key of "customer_id", and the "orders" table has a foreign key "customer_id" that references the primary key of the "customers" table. This relationship between the two tables ensures that every order in the "orders" table is associated with a specific customer in the "customers" table.

Most current database implementations are based on relational databases. Because
the relational model uses keys to create associations among tables, it is essential to learn the characteristics of good primary keys and how to select them. Selecting a good primary key is too important to be left to chance, so this chapter covers the critical aspects of primary key identification and placement. Focusing on practical database design, this chapter also illustrates some special design cases that highlight the importance of flexible designs, which can be adapted to meet the demands of changing data and information requirements.

Data modeling is a vital step in the development of databases that in turn provides a good foundation for successful application development. Remember that good database applications cannot be based on bad database designs, and no amount of outstanding coding can overcome the limitations of poor database design