NOT NULL

SQL NOT NULL Constraint

In SQL, the NOT NULL constraint is used to ensure that a column cannot contain any NULL values. When you define a column with the NOT NULL constraint, it means that every row inserted into the table must have a value for that column, and the value cannot be NULL.

Here's how you can define a column with the NOT NULL constraint

        CREATE TABLE table_name (
            column1 datatype NOT NULL,
            column2 datatype,
            ...
        );
      

In this syntax:

• table_name: The name of the table you are creating.
• column1: The name of the column.
• datatype: The data type of the column.
• NOT NULL: The constraint that specifies the column cannot contain NULL values.

Example:

Let's say we have a table named employees and we want to ensure that the first_name and last_name columns cannot contain NULL values:

        CREATE TABLE employees (
            employee_id INT PRIMARY KEY,
            first_name VARCHAR(50) NOT NULL,
            last_name VARCHAR(50) NOT NULL,
            salary DECIMAL(10, 2)
        );
    

In this example, both first_name and last_name columns are defined with the NOT NULL constraint. This means that every row inserted into the employees table must have a non-NULL value for both first_name and last_name.

If you try to insert a row into the employees table without providing values for first_name or last_name, or if you try to update an existing row to set either of these columns to NULL, you will receive an error due to the NOT NULL constraint violation.

Using the NOT NULL constraint helps enforce data integrity by ensuring that essential data is always present in the database, and it can also improve query performance by eliminating the need to check for NULL values in queries.