Question

What is NOT NULL constraint?

Answer 1

By default, a column can hold NULL values. The NOT NULL constraint enforces a column to NOT accept NULL values. This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field.

Answer 2

Explanation:

• A table's columns should never have the null value set to them by using the NOT NULL constraint.
• Any insert or update operation that tries to put a null value into a column that has a NOT NULL constraint declared for it will fail.
• The NOT NULL constraint makes it mandatory for a column to reject NULL entries.
• This forces a field to always have a value, so you can't add a new record or update an existing record without giving this field a value.
• For instance, the statement SELECT * FROM employees WHERE last name IS NOT NULL will retrieve all items from the workers table where the last name field does not contain a null value.

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