create table t1 (f1 varchar(128), f2 varchar(128), key (f2,f1) using btree) engine=memory;
insert into t1 values (null,'not'),('one',null),('two',null),('three','');
select * from t1 where f1 = 'one'  and f2 is null;
f1	f2
one	NULL
drop table t1;