MySQL即使连接数据为空也如何返回行 [英] Mysql how to return rows even if join data is empty

问题描述

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

I have three tables (product, product_description, product_store_description).

产品

此表有一行

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

产品说明

此表有零行

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

product_store_description

This table has zero rows

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

我想为所有表WHERE product.product_id = '1'返回.description列.

I would like to return the .description column for all tables WHERE product.product_id = '1'.

所以在这种情况下，我想要的输出将是

So in this case the output I would like would be

$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)

我已经尝试了许多SQL变体，这就是目前的样子.

I have tried many variations of my SQL and this is how it stands at the moment.

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

我搜索了一个导致我使用RIGHT JOIN的答案，但这并不能解决我的问题.

I have searched for an answer which has led me to use a RIGHT JOIN but this has not solved my problem.

我已经创建了示例表的 HERE 的SQL小提琴，但是我无法让任何人都可以上班.

Ive created a SQL fiddle HERE of the example tables, unfortunately I haven't been able to get anyone answer to work yet.

推荐答案

如果表product包含至少一个数据.并且您想要显示其他列NULL或自定义字符串(例如"---")，即使连接属性为null，也必须在查询中指定要连接的列.因此，以下查询将为您提供数据.

If table product is containing atleast one data. And You want to show other columns either NULL or custom string like "---" the you have to specify in query that join even if joining attribute is null. So following query will give you data.

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);

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