MySQL的自然加入不工作

我在mysql服务器中有两个表.我将这些表用于研究JOIN多个表,但似乎有些不正确：

mysql> select * from category;
+-------------+-----------+
| category_id | name      |
+-------------+-----------+
|           1 | fruit     |
|           2 | vegetable |
+-------------+-----------+
2 rows in set (0.00 sec)

mysql> desc category;
+-------------+-------------+------+-----+---------+----------------+
| Field       | Type        | Null | Key | Default | Extra          |
+-------------+-------------+------+-----+---------+----------------+
| category_id | int(11)     | NO   | PRI | NULL    | auto_increment |
| name        | varchar(50) | NO   |     | NULL    |                |
+-------------+-------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

和：

mysql> select * from goods;
+---------+--------+-------------+------+
| good_id | name   | category_id | cost |
+---------+--------+-------------+------+
|       1 | banan  |           1 | 1.00 |
|       2 | potato |           2 | 1.00 |
|       3 | peach  |           1 | 1.00 |
+---------+--------+-------------+------+
3 rows in set (0.00 sec)

mysql> desc goods;
+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| good_id     | int(11)      | NO   | PRI | NULL    | auto_increment |
| name        | varchar(100) | NO   |     | NULL    |                |
| category_id | int(11)      | NO   | MUL | NULL    |                |
| cost        | decimal(6,2) | NO   |     | NULL    |                |
+-------------+--------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

第二个表具有外键(category_id),我可以使用INNER JOIN将其加入：

mysql> select c.name category, g.name, g.cost from category as c INNER JOIN goods g ON c.category_id = g.category_id;
+-----------+--------+------+
| category  | name   | cost |
+-----------+--------+------+
| fruit     | banan  | 1.00 |
| vegetable | potato | 1.00 |
| fruit     | peach  | 1.00 |
+-----------+--------+------+
3 rows in set (0.00 sec)

我尝试使用NATURAL JOIN,但没有成功,看来我不知道为什么(((

mysql> select c.name, g.name, g.cost from category as c NATURAL JOIN goods g;
Empty set (0.00 sec)

有人可以解释为什么NA​​TURAL JOIN不起作用吗？

解决方法:

我遇到了完全相同的事情,而我的Google搜索使我想到了这个问题.我最终想通了,所以我想将答案发布在这里.

这是罪魁祸首：

Instead of specifying a join condition through ON, USING or a WHERE clause, the NATURAL keyword tells the server to match up any column names between the two tables, and automatically use those columns to resolve the join.

您的水果和类别表都有一个名为“名称”的列.当SQL尝试将两者结合时,它将尝试结合所有类似的列.因此,category_id == category_id,但name！= name.

重命名您的列tablename_column.

标签：inner-join,natural-join,mysql
来源： https://codeday.me/bug/20191028/1954856.html