Question

我有三个表：

产品

+----+-----------------------------------------------------+
| id | title                                               |
+----+-----------------------------------------------------+
|  1 | Lorem Ipsum is simply dummy text of the printing: 1 |
|  2 | Lorem Ipsum is simply dummy text of the printing: 2 |
|  3 | Lorem Ipsum is simply dummy text of the printing: 3 |
|  4 | Lorem Ipsum is simply dummy text of the printing: 4 |
|  5 | Lorem Ipsum is simply dummy text of the printing: 5 |
+----+-----------------------------------------------------+

货币：

+----+-------+-------+
| id | alias | title |
+----+-------+-------+
|  1 | JPY   | JPY   |
|  2 | USD   | USD   |
|  3 | GBP   | GBP   |
+----+-------+-------+

product_price：

+------------+-------------+-------------+
| product_id | currency_id | price       |
+------------+-------------+-------------+
|          1 |           1 | 3600000.000 |
|          1 |           2 |     160.000 |
|          2 |           1 | 3260000.000 |
+------------+-------------+-------------+

我想获取特殊产品的价格，如果表product_price包含产品价格，则在使用内部联接时很容易。但是，如果表product_price不包含产品价格，我想得到结果。

例如：在站点上，用户选择“ JPY”，因此我可以显示价格为3600000.000的产品1。但是当用户切换到“ GBP”时，由于没有价格，因此无法显示产品1。

我同时使用了左联接，左外部联接和交叉联接，但无法获得正确的结果。用户为产品1选择“ GBP”时，我的预期结果是：

+----+-----------------------------------------------------+-------------+----------------+-------+
| id | title                                               | currency_id | currency_alias | price |
+----+-----------------------------------------------------+--------------------------------------|
|  1 | Lorem Ipsum is simply dummy text of the printing: 1 |           3 | GBP            | NULL  |
+----+-----------------------------------------------------+-------------+----------------+-------+

请帮助我，谢谢！

Answer 1

嗯。认为您只想要left join：

select p.*, c.alias, pp.price
from product p left join
     product_price pp
     on pp.product_id = p.id left join
     currency c
     on pp.currency_id = c.id and c.alias = 'JPY';

这将返回所有价格为JPY的商品（如果有）。

Answer 2

您将必须使用CROSS JOIN生成产品和货币之间的所有可能组合。然后，您可以将这些组合用作派生表，并对联结表进行LEFT JOIN：

SELECT
  dt.*, pp.price
FROM 
(
  SELECT
    p.id AS product_id, 
    p.title AS product_tile, 
    c.id AS currency_id, 
    c.alias AS currency_alias, 
    c.title AS currency_title 
  FROM product AS p
  CROSS JOIN currency AS c
) AS dt 
LEFT JOIN product_price AS pp 
  ON pp.product_id = dt.product_id AND 
     pp.currency_id = dt.currency_id 
WHERE dt.currency_title = 'GBP'  -- You can provide currency filter here

如果表不包含值，MySQL联接返回null

2 个答案: