mysql left join

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)