SQL - NOT NULL
By default, a column of a table holds NULL values. The SQL NOT NULL constraint is used to enforce a column to NOT accept NULL values. This ensures the column to always have a value. This implies that a new record can not be inserted without providing a value to this column. Similarly, this column can not be updated with a NULL value.
SQL NOT NULL constraint with CREATE TABLE statement
The below mentioned SQL code creates a table called Employee which contains five columns: EmpID, Name, City, Age and Salary in which NOT NULL constraint is applied on columns EmpID, Name and City.
CREATE TABLE Employee ( EmpID INT NOT NULL, Name VARCHAR(255) NOT NULL, City VARCHAR(100) NOT NULL, Age INT, Salary DECIMAL(18,2) );
SQL NOT NULL constraint with ALTER TABLE statement
In the above example, a table called Employee is created in which column Salary does not have NOT NULL constraint. To enforce NOT NULL constraint on this column, the SQL code is given below:
ALTER TABLE Employee MODIFY Salary DECIMAL(18,2) NOT NULL;
Remove NOT NULL constraint
To drop the NOT NULL constraint from column City of above mentioned Employee table, the SQL code is given below:
ALTER TABLE Employee ALTER COLUMN City VARCHAR(100) NULL;