Why Relational Databases Use Primary Keys and Foreign Keys: A Comprehensive Guide

Mastery of SQL Server's foreign key management features is essential for anyone working with databases, whether in design or maintenance. This article provides a comprehensive instruction on creating foreign keys and effectively resolving any issues that may arise. A well-structured database and accurate data are the results of skillfully managing foreign keys.

Similar to the question: “Why do carpenters use hammers and screwdrivers to build walls”? The only valid answer is: ‘Because those are the correct tools to implement relationships in a relational database.

Primary and Foreign Keys in Relational Databases

In most relational databases, the link called the relationship is a separate database object that imposes a specific constraint on the child table. Most typically, this constraint says that the specific foreign key values for a row inserted into the child table must exist in the parent table. An implicit part of this constraint is that these values identify one row in the parent, and the parent table uses these values as its primary key. The use of the parent table's primary key ensures that exactly one parent exists for each child.

Relational databases regard the relationships between tables as table-level objects. Table-level objects have to be defined in structural, not value, terms. Therefore, the information required by a foreign key constraint must be specified at the structural level. That's why the columns that constitute the primary key of the parent are pushed into the child. The specific values a foreign key relation will validate for a given insert aren’t checked until that insert occurs, but the constraint that tells the database to make that check is created as part of the structure of the database.

Storing Primary Key Columns in the Child Table

There are several ways to store those primary key columns in the child table:

Identifying Relation: We can make them part of the child table's primary key. This is called an identifying relation. In an identifying relation, the primary key of the child table is composed of the foreign key value and some other columns that are unique to the child table. This design ensures that the foreign key relationship is meaningful and that the child table's data is tightly tied to the parent table. Non-identifying Relation: We can simply park them among the rest of the non-key attributes in the child table. This is called a non-identifying relation. In a non-identifying relation, the foreign key columns can be nullable or non-nullable. If the foreign key columns are not nullable, the relation is termed mandatory. A mandatory relation ensures that any child record must have a valid parent record, but it doesn't enforce the uniqueness of the child records relative to the parent.

Understanding and implementing relationships correctly in relational databases through primary and foreign keys is crucial for maintaining data integrity and ensuring that your database is well-structured and efficient.

Conclusion

In summary, primary keys and foreign keys play a vital role in relational databases. Primary keys ensure uniqueness and identity, while foreign keys enforce referential integrity. By mastering these concepts and their applications, you can achieve more reliable and efficient database management.