join SQL when join column contains null value

[yanhuqing666]

• **dble version：all **
• preconditions ：
  CREATE TABLE test1 (
  id bigint(11) NOT NULL,
  c_char char(255) DEFAULT NULL,
  PRIMARY KEY (id)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE test2 (
id bigint(11) NOT NULL,
c_char char(255) DEFAULT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert into test1 values(1,'1');
insert into test1(id) values(2);

insert into test2 values(1,'1');
insert into test2(id) values(2);

• configs：

schema.xml

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE dble:schema SYSTEM "schema.dtd">

<schema name="nosharding_test" sqlMaxLimit="100" dataNode="dn5">
		<table name="test1"  dataNode="dn5"  />
		<table name="test2"  dataNode="dn5"  />
</schema>

rule.xml

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE dble:rule SYSTEM "rule.dtd"><dble:rule xmlns:dble="http://dble.cloud/">

server.xml

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE dble:server SYSTEM "server.dtd">

<dble:server xmlns:dble="http://dble.cloud/">
<system>
...
<property name="useJoinStrategy">false</property>
</system>
...
</dble:server>

• steps：
  step1. select * from test1 a inner join test2 b on a.c_char =b.c_char;
  step2. select * from test1 a right join test2 b on a.c_char =b.c_char;
  step3. select * from test1 a left join test2 b on a.c_char =b.c_char;
• expect result：
  1.

+----+--------+----+--------+
| id | c_char | id | c_char |
+----+--------+----+--------+
|  1 | 1      |  1 | 1      |
+----+--------+----+--------+
1 row in set (0.05 sec)

+------+--------+----+--------+
| id   | c_char | id | c_char |
+------+--------+----+--------+
|    1 | 1      |  1 | 1      |
| NULL | NULL   |  2 | NULL   |
+------+--------+----+--------+
2 rows in set (0.13 sec)

+----+--------+------+--------+
| id | c_char | id   | c_char |
+----+--------+------+--------+
|  1 | 1      |    1 | 1      |
|  2 | NULL   | NULL | NULL   |
+----+--------+------+--------+
2 rows in set (0.06 sec)

• real result：

Empty set (0.02 sec)

+----+--------+----+--------+
| id | c_char | id | c_char |
+----+--------+----+--------+
| NULL | NULL   |  2 | NULL   |
| NULL | NULL   |  1 | 1      |
+----+--------+----+--------+
2 rows in set (0.01 sec)

+----+--------+----+--------+
| id | c_char | id | c_char |
+----+--------+----+--------+
|  2 | NULL   | NULL | NULL   |
|  1 | 1      | NULL | NULL   |
+----+--------+----+--------+
2 rows in set (0.02 sec)

• supplements：
  1.

/label ~BUG

[yanhuqing666]

may import by #140