When the table does not take the null value but you want to enter the null value in a particular column then you have to run this query..

alter table <table_name> modify column <column_name> <data_type>(value) null;

If you want that the column does not contain the null value but the column is nullable then modify the column by this query

alter table <table_name> modify column <column_name> <data_type>(value)not null;