NOT NULL integrity constraints
The NOT NULL constraint can restrict a column from containing NULL values.
> column_name data_type NOT NULL;
You cannot insert a
NULL value into a column that contains a
NOT NULL constraint or update an old value to
create table t1(a int not null,b int); mysql> insert into t1 values(null,1); ERROR 3819 (HY000): constraint violation: Column 'a' cannot be null mysql> insert into t1 values(1,null); Query OK, 1 row affected (0.01 sec) mysql> update t1 set a=null where a=1; ERROR 3819 (HY000): constraint violation: Column 'a' cannot be null
Example Explanation: In the above example, because there is a non-null constraint in column a, the execution of the first insert statement will fail, the second statement satisfies the non-null constraint in column a, and there is no non-null constraint in column b so that it can be inserted successfully. The update statement fails because it triggers the non-null constraint of column a.