MySQL Database Management System > MySQL Statements and Clauses

MySQL - NULL and NOT NULL Clauses

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NOT NULL indicates that the column cannot have a NULL value for any record; NULL indicates that NULL is a valid value (even when the column has a foreign key constraint).

NULL is defined as "a missing unknown value," and it is treated differently than other values. Because the result of any arithmetic comparison with NULL is also NULL, such comparisons yield no meaningful results. In MySQL, false is 0 or NULL, and true is anything else.

NOT NULL indicates that the column cannot have a NULL value for any record; NULL indicates that NULL is a valid value (even when the column has a foreign key constraint).

In MySQL, you use the ALTER TABLE MODIFY command to remove a NOT NULL constraint for a column and then restate the column definition, eliminating the NOT NULL attribute.

NULL is defined as "a missing unknown value," and it is treated differently than other values. Because the result of any arithmetic comparison with NULL is also NULL, such comparisons yield no meaningful results. In MySQL, false is 0 or NULL, and true is anything else.

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