看不懂mysql自左连接查询

我遇到一个问题，它是，

SELECT it1.survey_set_id, it1.type, it1.value FROM survey_condition_filter it1 LEFT JOIN survey_condition_filter it2 ON(it1.survey_set_id = it2.survey_set_id AND it2.type = 3002) WHERE it1.type IN (2000, 2001, 2002) AND it2.value IS NULL;

为什么在上面的查询中使用自左联接.

SELECT survey_set_id, type, value FROM survey_condition_filter WHERE type IN (2000, 2001, 2002);

上面的查询不是等同于第一个使用自左联接的查询吗？因为查询只过滤了IN (2000, 2001, 2002) AND it2.value IS NULL.我在这里混淆了连接查询的用法，并且不能真正理解第一个查询的工作原理.

这是survey_condition_filter号桌

+---------------+--------------+------+-----+---------+-------+
| Field         | Type         | Null | Key | Default | Extra |
+---------------+--------------+------+-----+---------+-------+
| survey_id     | bigint       | NO   | PRI | NULL    |       |
| survey_set_id | bigint       | NO   | PRI | NULL    |       |
| type          | int          | NO   | PRI | NULL    |       |
| condition     | tinyint      | NO   | PRI | NULL    |       |
| value         | varchar(15)  | NO   | PRI | NULL    |       |
| display_value | text         | NO   |     | NULL    |       |
| order         | int          | YES  |     | NULL    |       |
| created_at    | datetime     | NO   |     | NULL    |       |
| created_by    | varchar(255) | YES  |     | NULL    |       |
| updated_at    | datetime     | NO   |     | NULL    |       |
| updated_by    | varchar(255) | YES  |     | NULL    |       |
| deleted_at    | datetime     | YES  |     | NULL    |       |
| deleted_by    | varchar(255) | YES  |     | NULL    |       |
+---------------+--------------+------+-----+---------+-------+