NOT NULL constraint in SQL - Cheat Sheet & Quick Revision

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beginner

What does the NOT NULL constraint do in a database?

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

beginner

How do you add a NOT NULL constraint to a column when creating a table?

Use column_name datatype NOT NULL in the CREATE TABLE statement.

intermediate

Can a column with a NOT NULL constraint accept empty strings ('')?

Yes, NOT NULL only prevents NULL values, but empty strings are considered valid values.

beginner

What happens if you try to insert a row without a value for a NOT NULL column?

The database will reject the insert and return an error because the column requires a value.

intermediate

How can you add a NOT NULL constraint to an existing column?

Use an ALTER TABLE statement like ALTER TABLE table_name ALTER COLUMN column_name SET NOT NULL; (syntax may vary by SQL dialect).

What does the NOT NULL constraint prevent?

A Deleting rows

B Inserting duplicate values

C Inserting NULL values into a column

D Updating values

The NOT NULL constraint stops NULL values from being inserted into the column.

Which SQL keyword is used to define a column that cannot be NULL?

A UNIQUE

B CHECK

C PRIMARY KEY

D NOT NULL

The NOT NULL keyword specifies that the column must have a value.

If a column is defined as NOT NULL, what happens if you try to insert a row without a value for that column?

A The insert fails with an error

B The database inserts a default value automatically

C The row is inserted with an empty string

D The row is inserted with NULL

The database rejects the insert because the column requires a non-NULL value.

Can a NOT NULL column contain an empty string ('')?

A Only if the column is numeric

B Yes, empty strings are allowed

C No, empty strings are treated as NULL

D Only if the column is a primary key

Empty strings are valid values and different from NULL, so they are allowed.

How do you add a NOT NULL constraint to an existing column in most SQL databases?

A ALTER TABLE table_name ALTER COLUMN column_name SET NOT NULL;

B ALTER TABLE table_name MODIFY column_name NOT NULL;

C ALTER TABLE table_name ADD NOT NULL column_name;

D ALTER TABLE table_name CHANGE column_name NOT NULL;

The standard way is to use ALTER COLUMN column_name SET NOT NULL, but syntax can vary by database.

Explain what the NOT NULL constraint does and why it is useful in a database.

Think about what happens if a column must always have a value.

Describe how you would add a NOT NULL constraint to a column in an existing table.

Consider how to change a column's rules after the table is created.