即使连接数据为空，Mysql如何返回行

Question

我有三个表 ( product, product_description, product_store_description)。

产品

这个表只有一行

+----------+---------------------+
|product_id|      description    |
+----------+---------------------+
|    1     |'regular description'|
+----------+---------------------+

产品描述

此表有零行

+----------+-----------+--------+
|product_id|description|language|
+----------+-----------+--------+

product_store_description

This table has zero rows

+----------+-----------+--------+--------+
|product_id|description|language|store_id|
+----------+-----------+--------+--------+

我想返回.description所有表的列WHERE product.product_id = '1'。

所以在这种情况下，我想要的输出是

$row = $result->fetch_assoc();
echo '<pre>';
var_dump($row);
echo '</pre>';

array (size=3)
    'p_description' => string 'regular description' (length=19)
    'pd_description' => string '' (length=0)
    'psd_description' => string '' (length=0)

我已经尝试了我的 SQL 的许多变体，这就是目前的情况。

SELECT p.description AS p_description ,pd.description AS pd_description, psd.description AS psd_description FROM product AS p 
RIGHT JOIN product_description AS pd ON(pd.product_id = p.product_id)
RIGHT JOIN product_store_description AS psd ON(psd.product_id = p.product_id) 
WHERE p.product_id = '1'
AND pd.language = 'en'
AND psd.language = 'en' AND psd.store_id = 1;

这将返回零行

我已经搜索了一个导致我使用 a 的答案，RIGHT JOIN但这并没有解决我的问题。

我在此处创建了示例表的 SQL 小提琴，不幸的是，我还没有让任何人回答工作。

Answer 1

如果表product包含至少一个数据。并且您想显示其他列NULL或自定义字符串，例如"---"您必须在查询中指定的连接，即使 join 属性为空。因此，以下查询将为您提供数据。

SELECT
p.description AS p_description ,ifnull(pd.description,"---") AS pd_description,
ifnull(psd.description,"--") AS psd_description
FROM product AS p
LEFT JOIN Product_description AS pd ON(pd.product_id = p.product_id)
LEFT JOIN product_store_description AS psd ON(psd.product_id = p.product_id)
WHERE p.product_id = '1' AND (pd.language = 'en' or pd.language is null)
AND (psd.language = 'en' or psd.language is null) AND 
(psd.store_id = 1 or psd.store_id is null);