FOREIGN KEY constraint in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a FOREIGN KEY constraint in a database?

A FOREIGN KEY constraint is a rule that links one table's column to another table's primary key column. It ensures that the value in the foreign key column matches a value in the referenced table, keeping data consistent.

beginner

Why do we use FOREIGN KEY constraints?

We use FOREIGN KEY constraints to keep data accurate and connected. They prevent adding values in one table that don't exist in the related table, helping avoid mistakes and keeping relationships clear.

intermediate

What happens if you try to insert a value in a FOREIGN KEY column that does not exist in the referenced table?

The database will reject the insert and show an error because the FOREIGN KEY constraint requires the value to exist in the referenced table to keep data consistent.

intermediate

How do you define a FOREIGN KEY constraint when creating a table?

You define a FOREIGN KEY constraint by specifying the column and the referenced table and column using the syntax: FOREIGN KEY (column_name) REFERENCES referenced_table(referenced_column).

intermediate

Can a FOREIGN KEY column contain NULL values?

Yes, a FOREIGN KEY column can contain NULL values unless you specify NOT NULL. NULL means no link to another table, which is allowed in many cases.

What does a FOREIGN KEY constraint do?

A Creates a unique index on a column

B Links a column in one table to a primary key in another table

C Deletes all rows in a table

D Changes the data type of a column

A FOREIGN KEY constraint links a column in one table to a primary key in another table to maintain data consistency.

What happens if you insert a value in a FOREIGN KEY column that does not exist in the referenced table?

A The database rejects the insert with an error

B The value is converted to NULL

C The referenced table is automatically updated

D The insert succeeds without any error

The database rejects the insert because the FOREIGN KEY constraint requires the value to exist in the referenced table.

Which SQL keyword is used to define a FOREIGN KEY constraint?

A CHECK

B PRIMARY KEY

C UNIQUE

D FOREIGN KEY

FOREIGN KEY is the keyword used to define a foreign key constraint linking tables.

Can a FOREIGN KEY column have NULL values?

A No, it must always have a value

B Only if the referenced table allows NULL

C Yes, unless NOT NULL is specified

D Only if the column is a primary key

A FOREIGN KEY column can have NULL values unless it is declared NOT NULL.

What is the main purpose of a FOREIGN KEY constraint?

A To enforce referential integrity between tables

B To store large text data

C To speed up queries

D To create backup copies of data

The main purpose of a FOREIGN KEY constraint is to enforce referential integrity between tables.

Explain what a FOREIGN KEY constraint is and why it is important in databases.

Think about how tables stay connected and data stays accurate.

Describe how to create a FOREIGN KEY constraint when making a new table in SQL.

Remember the syntax FOREIGN KEY (column) REFERENCES table(column).