Primary key vs. foreign key: How do these differ?

Your relational database management system (DBMS) works like a busy supply chain where raw data comes in from different places and gets loaded into tables. But without a way to identify and link items across your infrastructure, tables become isolated and turn into inefficient data silos.

Keys build the necessary connections between relations in a database to prevent this fragmentation and keep everything logically connected. There are many different types of keys that manage relations in a database, such as candidate and composite key options. However, most database table logic relies on two fundamental pillars: primary and foreign keys. To use them effectively, you need a clear understanding of each.

Learn how primary keys vs. foreign keys enforce referential integrity, how to use them to build a resilient database from the ground up, and why relational databases use primary keys and foreign keys.

What is a primary key in a database?

A primary key (PK) is a unique identifier for every single row in a database table. Just as a Social Security Number ensures no two citizens get mistaken for each other in a national database, a PK ensures no two records are identical. DBMSs establish this constraint to quickly locate, update, or query specific data without risking overlap.

Advantages of a primary key

Implementing a PK in a database provides the structural background for data integrity and efficient data management. Here are the main benefits:

• Guarantees unique identification: In a relational database, assigning every record in the table with a distinct value that uniquely identifies the entry keeps data clean from the start and prevents potential corruption of analytical models.
• Enhances query performance: Defining the PK prompts the database to create a clustered index for the PK column. This optimization makes it easy for the database engine to locate a specific ID or attribute without performing a full table scan, leading to faster queries.
• Ensures data integrity: Implementing PKs invalidates the possibility of a NULL value. This constraint ensures critical records always have a unique and valid ID to maintain the reliability of the schema as you scale data volume.

Disadvantages of a primary key

Despite their role in ensuring data integrity, PKs come with a few trade-offs that teams should account for during the design phase:

• Structural rigidity: Once you establish the PK and other tables reference it, changing it becomes a big migration hurdle. Changing the PK logic often means altering table structures across the entire relational database, which can lead to downtime.
• Complexity with composite keys: Where a single column isn’t enough to guarantee uniqueness, teams may need to add a composite key. These multi-column keys make SQL joins more complex and increase the effort required to manage or insert new row entries as you scale the database.
• Indexing and write overload: While a PK speeds up reads, it adds a small overhead to write operations. Every time you insert data, the database must verify uniqueness and update the index, which adds time to each write.

What is a foreign key in a database?

While a PK identifies the record, a foreign key (FK) identifies that record’s relationship to other tables. Technically, an FK is one column or a group of columns in a database table that connects data in two tables by cross-referencing the other table’s PK.

For example, in an e-commerce database, an “Orders” table might have an “Order ID” as its PK, but it’ll also include a “CustomerID” column. That “CustomerID” is an FK in the “Orders” table because it points back to the PK in the “Customers” table. This connection simplifies storing data across multiple tables without the need to duplicate related information.

This connection makes it simple to store data across multiple tables without duplicating related information.

Advantages of a foreign key

FKs keep relational database architectures clean. Core benefits include:

• Enforces referential integrity: FKs prevent the creation of potential “orphan” records because each record added to the child table must correspond to the ID in the parent table. This constraint maintains a consistent relationship between tables.
• Simplifies data standardization: As illustrated in the example above, there’s no need to repeat the same information across tables when you can simply connect them. FKs let teams store related data separately and link it rather than duplicating values.
• Automates maintenance with cascade: Deleting a parent record deletes the associated records without any manual intervention, keeping records clean and compliant with data privacy laws like the General Data Protection Regulation.

Disadvantages of a foreign key

PKs and FKs work well together in SQL, but there are some operational costs that teams should consider when implementing them:

• Performance overhead during writes: Like PKs, there’s an overhead associated with inserting a new row with an FK. The database must perform a lookup in the parent table to verify the value exists, causing a lag in large databases.
• Cascading complications: A clean delete is useful when it’s intentional, but problematic when it isn’t. Accidentally deleting one record in a parent table would also wipe out all associated data, and it might not be easy to get the data back.
• Complexity in data migrations: FKs create dependencies between data in two tables. This adds extra steps to ETL workflows and database management because you must either load data in a specific order — parents before children — or temporarily disable constraints and then re-enable them, increasing the risk of errors during migration.

Key differences between a primary key and a foreign key

While both keys are essential for data integrity, there’s a big difference in how they serve database architecture. While a PK ensures internal identity, FKs facilitate connectivity. Here’s how they differ in relational database management.

Use cases: When to use a primary key vs. a foreign key

Deciding when to use a PK or FK depends on how you choose to organize data and relational databases. The PK sits at the center and everything else points back to it.

When to use a primary key

Use a PK when you need one “source of truth” for a specific object. For example, the “Order_ID” is the likely “source of truth” in an “Orders” table. Setting “Order_ID” as the PK ensures that “Order A” and “Order B” are different records, even if one person bought the same items twice on the same day.

Implementing a PK also prevents duplicates by blocking redundant data at the entry point. The constraints associated with using a PK prevent the system from creating two separate accounts for the same person in a “Users” table. This is why you can’t have two Amazon accounts with the same email address.

When to use a foreign key

The main purpose of an FK is to link tables by connecting an activity to an identity. For example, the FK connects multiple orders to the same person or even the same location. This is instrumental in finding patterns for better inventory management or customer satisfaction.

FKs are also crucial for looking up reference data by pulling attributes from a central “lookup” table. For example, instead of manually entering “International-Priority” for every overseas shipment, you could use a “Shipping Code” FK that also connects to a “Services_Levels” table.

Ensure data integrity with Fivetran

PKs and FKs ensure data connectivity and consistency in relational databases, but things get complicated when it’s time to migrate your database or records to a new storage or hosting location.

Keys enforce integrity at rest, but moving data at scale threatens that structure. Fivetran Governance preserves your source schemas and constraints during automated data migrations, so the PK and FK relationships remain intact.

Book a demo now to see it in action.

FAQ

What are the types of primary keys?

There are three types of primary keys:

• Natural keys: Use a value that already exists in the real world, such as a Social Security Number or an email address.
• Surrogate keys: Use a value generated by a computer for the DMBS, such as an order number at a retail store or the confirmation number for a flight booking.
• Composite keys: Use values from two columns to create a unique identifier when one column is not enough, such as warehouse ID combined with the product SKU to track an item’s location.

Can a table have multiple primary keys?

No, a table can’t have multiple primary keys. Each table has only one primary key. However, that single key could be a composite of multiple columns forming one unique identifier.

Can a foreign key be a primary key?

Yes, a foreign key can also be a primary key. Junction tables bridge many-to-many relationships, so a column could be part of a composite primary key in its own table while serving as a foreign key that points to another table.

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