MySQL左连接不同于简单连接。MySQL LEFT JOIN提供该表额外字段在左侧。如果使用LEFT JOIN,得到的所有记录的匹配方式相同,在左边表中得到的每个记录不匹配也会有一个额外的记录。从而确保(在本例子),每次作者信息都会列出:SELECT * from tutorials_tbl;+-------------+----------------+-----------------+-----------------+| tutorial_id | tutorial_title | tutorial_author | submission_date |+-------------+----------------+-----------------+-----------------+ | | |+-------------+----------------+-----------------+-----------------+mysql> SELECT * FROM tcount_tbl;+-----------------+----------------+| tutorial_author | tutorial_count |+-----------------+----------------+ || mahnaz | NULL || Jen | NULL | | | |+-----------------+----------------+mysql> SELECT a.tutorial_id, a.tutorial_author, b.tutorial_count -> FROM tutorials_tbl a LEFT JOIN tcount_tbl b -> ON a.tutorial_author = b.tutorial_author;+-------------+-----------------+----------------+| tutorial_id | tutorial_author | tutorial_count |+-------------+-----------------+----------------+ | | Abdul S | NULL | |+-------------+-----------------+----------------+ rows in set (0.02 sec)