drop table if exists t1,t2,t5,t6;
CREATE TABLE t1 (
pk int primary key,
id int,
gender varchar(1)
);
CREATE TABLE t2 (
user_id int primary key,
birthday datetime
);
insert into t1 values (1, NULL, 'M'), (2, 1, 'M'), (3, 2, 'F'),(4, 3, 'F'),(5, 4, 'F'),(6, 5, 'M');
insert into t2 values (1, '2002-06-09 00:00:00'),(2, '2002-06-09 00:00:00'),(100, '2002-06-09 00:00:00'),
(3, '2002-06-09 00:00:00'),(4, '2002-06-09 00:00:00');
select  id,gender,user_id from  t1,t2  where t2.user_id=t1.id;
id	gender	user_id
1	M	1
2	F	2
3	F	3
4	F	4
create table t5(a int);
create table t6(a int);
insert into t5 values (null), (null);
insert into t6 values (null), (null);
select * from t5, t6 where t5.a <=> t6.a;
a	a
NULL	NULL
NULL	NULL
NULL	NULL
NULL	NULL
drop table t1,t2,t5,t6;