Foreign key linking mental model in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a foreign key in a database?

A foreign key is a column or set of columns in one table that refers to the primary key in another table. It creates a link between the two tables.

beginner

Why do we use foreign keys?

Foreign keys help keep data consistent by ensuring that the value in one table matches a value in another table. This prevents errors like referencing a non-existent record.

intermediate

How does a foreign key create a mental model of linking tables?

Think of a foreign key as a bridge connecting two tables. It helps you understand how data in one table relates to data in another, like linking a customer to their orders.

intermediate

What happens if you try to insert a foreign key value that doesn't exist in the referenced table?

The database will reject the insert because it violates the foreign key constraint. This keeps data accurate and linked properly.

intermediate

Can a foreign key be null? What does that mean?

Yes, a foreign key can be null if the relationship is optional. It means the record doesn't currently link to any record in the other table.

What does a foreign key in a table do?

A Stores duplicate data

B Creates a new table

C Deletes data from another table

D Links to a primary key in another table

A foreign key links to a primary key in another table to create a relationship.

What happens if you insert a foreign key value that does not exist in the referenced table?

A The database creates a new record automatically

B The insert is rejected

C The foreign key value is ignored

D The database crashes

The database rejects the insert to maintain data integrity.

Which of these best describes a foreign key?

A A unique identifier for a table

B A column that stores text data

C A link to another table's primary key

D A way to speed up queries

A foreign key links to another table's primary key to connect data.

Can a foreign key column contain NULL values?

A Yes, if the relationship is optional

B No, it must always have a value

C Only if the primary key is NULL

D Only in the primary table

Foreign keys can be NULL to indicate no current link.

What is the main purpose of a foreign key constraint?

A To enforce data consistency between tables

B To speed up data entry

C To allow duplicate records

D To delete related records automatically

Foreign key constraints ensure data stays consistent and linked.

Explain in your own words how a foreign key links two tables in a database.

Think about how one table points to another like a bridge.

Describe what happens if you try to add a record with a foreign key value that does not exist in the referenced table.

Consider the database's role in keeping data accurate.