Question

我有两个表：产品和元。

产品表：

+----+----------+
| id | name     |
+----+----------+
|  1 | TV       |
|  2 | Computer |
|  3 | Freezer  |
+----+----------+

元表：

+----+------------+-----------+------------+
| id | product_id | meta_key  | meta_value |
+----+------------+-----------+------------+
|  1 |          1 | currency  | USD        |
|  2 |          1 | price     | 1100       |
|  3 |          2 | currency  | PLN        |
|  4 |          2 | price     | 9300       |
|  5 |          3 | currency  | USD        |
|  6 |          3 | price     | 1200       |
+----+------------+-----------+------------+

现在以下查询正常工作：

select price.product_id, products.name, price.meta_value as 'price', currency.meta_value as 'currency' 
from meta as price
join meta as currency on(price.product_id=currency.product_id and currency.meta_key='currency')
join products on(products.id=price.product_id)
where price.meta_key='price';

结果：

+------------+----------+-------+----------+
| product_id | name     | price | currency |
+------------+----------+-------+----------+
|          1 | TV       | 1100  | USD      |
|          2 | Computer | 9300  | PLN      |
|          3 | Freezer  | 1200  | USD      |
+------------+----------+-------+----------+

但查询：

select price.product_id, products.name, price.meta_value as 'price', currency.meta_value as 'currency' 
from meta as price, meta as currency
join products on(products.id=price.product_id)
where
    price.product_id=currency.product_id
    and price.meta_key='price'
    and currency.meta_key='currency';

返回：＆＃34;未知列＆＃39; price.product_id＆＃39;在＆＃39; on＆＃39;＆＃39;＆＃34;

为什么会这样？

Answer 1

您的“from”子句被解释为：

from meta as price, (meta as currency join products on (products.id = price.product_id)

因此，on子句没有price.product_id可用，因为它只知道meta as currency和products表。

将自联接表加入另一个表

1 个答案: