mysql サーバーに 2 つのテーブルがあります。これらのテーブルを使用して JOIN 複数のテーブルを学習しますが、何かが間違っているようです:
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)
と：
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)
2 番目のテーブルには外部キー (category_id) があり、INNER JOIN を使用してそれらを結合できます。
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)
NATURAL JOIN を使用しようとしましたが、うまくいかず、理由がわからないようです(((
mysql> select c.name, g.name, g.cost from category as c NATURAL JOIN goods g;
Empty set (0.00 sec)
NATURAL JOIN が機能しない理由を誰か説明できますか?