NOT NULL Constraint
NOT NULL Constraint
By default, a column can hold NULL values. If you do not want a column to have a NULL value, then you need to define such constraint on this column specifying that NULL is now not allowed for that column.A NULL is not the same as no data, rather, it represents unknown data.
Example
The following SQL creates a new table called CUSTOMERS, two of which, ID and NAME, specify not to accept NULLs:CREATE TABLE CUSTOMERS(
ID INT NOT NULL,
NAME VARCHAR (20) NOT NULL,
ADDRESS CHAR (25) ,
PRIMARY KEY (ID)
);
ALTER TABLE CUSTOMERS
MODIFY SALARY DECIMAL (18, 2) NOT NULL;
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