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.
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.
Introduction to SQL NOT NULL constraint
The NOT NULL constraint is a column constraint that defines the rule which constrains a column to have non-NULL values only.
ALTER TABLE NOT NULL statement
Typically, we define the NOT NULL constraints for columns when we create the table. However, sometimes, we want to change the constraint of a column that accepts a NULL value to not accept a NULL value.
What is SQL constraint?
SQL Constraints are rules used to limit the type of data that can go into a table, to maintain the accuracy and integrity of the data inside table. Column level constraints: Limits only column data.
What is a primary key constraint?
Primary key constraint uniquely identifies each record in a database. A Primary Key must contain unique value and it must not contain null value. Usually Primary Key is used to index the data inside the table.
What is a foreign key in math?
Foreign Key is used to relate two tables. The relationship between the two tables matches the Primary Key in one of the tables with a Foreign Key in the second table.
Can a column hold a null value?
By default, a column can hold NULL values. If you do not want a column to have a NULL value, use the NOT NULL constraint. It restricts a column from having a NULL value. We use ALTER statement and MODIFY statement to specify this constraint.
