Not-Null Constraint

What Does Not-Null Constraint Mean?

The not-null constraint is a restriction placed on a column in a relational database table. It enforces the condition that, in that column, every row of data must contain a value – it cannot be left blank during insert or update operations. If this column is left blank, this will produce an error message and the entire insert or update operation will fail.


Techopedia Explains Not-Null Constraint

Consider a table named CUSTOMER_MASTER that stores customer details for a bank’s database. Every customer must have at least a surname and must belong to a certain gender. The two columns dealing with surname and gender can then be marked as “NOT NULL” when creating the Customer_Master table.

A sample SQL script for doing this is given below:
CREATE TABLE customer_master (
surname CHAR NOT NULL,
firstname CHAR,
date_of_birth DATE NOT NULL,

The not-null constraint is a useful tool for database designers for enforcing business logic. Instead of using programming code to enforce the “must always have a value” concept, they simply use a built-in database feature.

Sometimes the not-null constraint is implicit. When a column is marked as a primary key (see the “custid” column in our example above), it is used to uniquely identify all rows in the table so that they obviously cannot be left blank.


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Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.