NOT NULL constraint behavior in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the NOT NULL constraint do in a database table?

It ensures that a column cannot have a NULL value, meaning every row must have a valid value for that column.

beginner

Can you insert a row with a NULL value in a column that has a NOT NULL constraint?

No, the database will reject the insert and return an error because the NOT NULL constraint requires a value.

intermediate

How does the NOT NULL constraint help maintain data quality?

By preventing missing or unknown values in important columns, it ensures data completeness and reliability.

intermediate

What happens if you try to update a NOT NULL column to NULL?

The update will fail with an error because the NOT NULL constraint forbids NULL values in that column.

advanced

Is it possible to add a NOT NULL constraint to an existing column that already contains NULL values?

No, you must first update or remove NULL values before adding the NOT NULL constraint, or the database will reject it.

What will happen if you insert a NULL value into a NOT NULL column?

A The insert will succeed with NULL stored

B The NULL will be ignored and default value used

C The NULL will be converted to zero

D The insert will fail with an error

The NOT NULL constraint forbids NULL values, so the insert fails with an error.

Which of the following best describes the NOT NULL constraint?

A Prevents NULL values in a column

B Allows NULL values in a column

C Automatically fills NULL values

D Deletes rows with NULL values

NOT NULL constraint prevents NULL values in the specified column.

Can you add a NOT NULL constraint to a column that currently has NULL values without changing the data?

A No, you must first remove or update NULL values

B Yes, always

C Yes, but only if the table is empty

D No, constraints cannot be added after table creation

You must remove or update NULL values before adding a NOT NULL constraint.

What is the main benefit of using NOT NULL constraints?

A Improves query speed

B Ensures data completeness

C Allows flexible data entry

D Automatically indexes the column

NOT NULL constraints ensure that important columns always have data, improving completeness.

If a column has a NOT NULL constraint, what happens when you try to update its value to NULL?

A The update succeeds and sets NULL

B The update sets the default value instead

C The update fails with an error

D The update is ignored silently

NOT NULL constraint forbids NULL values, so updating to NULL causes an error.

Explain what the NOT NULL constraint does and why it is important in databases.

Think about how missing data can affect your information.

Describe the steps needed to add a NOT NULL constraint to an existing column that currently has NULL values.

You cannot add NOT NULL if NULLs exist already.