我使用MySQL的ER模型创建了一个真实的表集。我试图进行连接，但似乎要么缺少某些东西，要么没有正确地执行它们以生成NULL值。以下是我创建的表格：
+----------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------+-------------+------+-----+---------+-------+
| FilmNumber | char(3) | NO | PRI | NULL | |
| FilmName | varchar(60) | YES | | NULL | |
| OpeningDate | varchar(20) | YES | | NULL | |
| TopBilledActor | char(60) | YES | | NULL | |
| Genre | varchar(20) | YES | | NULL | |
+----------------+-------------+------+-----+---------+-------+
+------------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------+-------------+------+-----+---------+-------+
| CriticSiteNumber | char(3) | NO | PRI | NULL | |
| CriticSiteName | varchar(30) | YES | | NULL | |
| CriticName | char(50) | YES | | NULL | |
+------------------+-------------+------+-----+---------+-------+
+------------------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------+---------+------+-----+---------+-------+
| FilmNumber | char(3) | YES | MUL | NULL | |
| CriticSiteNumber | char(3) | YES | MUL | NULL | |
| OverallScore | int(5) | YES | | NULL | |
+------------------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)
我的问题是，为了产生NULL值我需要什么类型的连接查询，或者我的表中的某些东西需要修改，因为我尝试的每个查询都不返回这些值。