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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)
);
If CUSTOMERS table has already been created, then to add a NOT NULL constraint to SALARY column in Oracle and MySQL:
ALTER TABLE CUSTOMERS
   MODIFY SALARY  DECIMAL (18, 2) NOT NULL;
Read more: NOT NULL Constraint
NULL-values constraints NOT-NULL-constraint

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