A NOT NULL constraint on a column does not allow to enter a NULL value. 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.
By default, every column can hold a NULL value.
NOT NULL Constraint on CREATE TABLE
In the below table tblEmployee EmpID, EmpFirstName, EmpLastName column values are mandatory because of NOT NULL constraint.
The EmpID column is a PRIMARY KEY column which does not accept NULL values and EmpFirstName and EmpLastName has NOT NULL constraints, therefore these columns does not accept NULL values.
CREATE TABLE tblEmployee
(
EmpID INT PRIMARY KEY,
EmpFirstName NVARCHAR(20) NOT NULL,
EmpLastName NVARCHAR(20) NOT NULL,
EmpEmail NVARCHAR(20),
EmpPhone NVARCHAR(15),
EmpDeptID INT,
EmpSalary INT,
EmpJoiningDate DATETIME
)
NOT NULL Constraint on ALTER TABLE
ALTER TABLE tblEmployee
MODIFY EmpSalary INT NOT NULL
Now, If I try to insert NULL value for EmpLastName, I will get an error as shown in the below image.

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