mysql - mysql查询中缺少数据，内部连接需要条件语句

Question

当我运行以下查询时，我一直遇到丢失数据的问题。 有些产品有特价，存储在表 oc_product_special 中，有些产品有正常价格存储在表 oc_product 中。
我发现它只在表oc_product_special 中有特价时才显示数据。它省略了没有特价而只有正常价格的任何数据。我不确定如何解决这个问题。我可以如何以及在何处添加条件语句或类似

if there is a regular price present and no special price then show regular price and 0 for special price.

<pre><code>SELECT       
        pd.name AS 'Product Name', 
        p.model AS UPC,  
        p.quantity AS 'Quantity', 
        p.price AS 'Regular Price', 
        ps.price AS 'Special Price', 
        p.cost AS 'COST', 
        p.status AS 'Status'
FROM oc_product p
    INNER JOIN oc_product_description pd
        ON pd.product_id = p.product_id
    INNER JOIN oc_product_special ps
        ON ps.product_id = p.product_id
    INNER JOIN oc_manufacturer m
        ON p.manufacturer_id = m.manufacturer_id
    INNER JOIN oc_product_to_category p2c 
        ON p2c.product_id = p.product_id
    INNER JOIN oc_category c 
        ON c.category_id = p2c.category_id
    INNER JOIN oc_category_description cd 
        ON c.category_id = cd.category_id
WHERE 
      c.category_id = 37 OR c.parent_id = 37
GROUP BY pd.name ORDER BY m.name ASC 
</code></pre>

<p></p>

Answer 1

使用 LEFT JOIN，它将保留连接左侧的记录，即使它们与右侧的任何内容都不匹配也是如此:

SELECT COALESCE(pd.name, 'NA') AS 'Product Name', 
       p.model AS UPC,  
       p.quantity AS 'Quantity', 
       p.price AS 'Regular Price', 
       COALESCE(ps.price, 0.0) AS 'Special Price', 
       p.cost AS 'COST', 
       p.status AS 'Status'
FROM oc_product p
LEFT JOIN oc_product_description pd
    ON pd.product_id = p.product_id
LEFT JOIN oc_product_special ps
    ON ps.product_id = p.product_id
INNER JOIN oc_manufacturer m
    ON p.manufacturer_id = m.manufacturer_id
INNER JOIN oc_product_to_category p2c 
    ON p2c.product_id = p.product_id
INNER JOIN oc_category c 
    ON c.category_id = p2c.category_id
INNER JOIN oc_category_description cd 
    ON c.category_id = cd.category_id
WHERE c.category_id = 37 OR
      c.parent_id = 37
GROUP BY pd.name
ORDER BY m.name

解释:

在 LEFT JOIN 中，当连接左侧的记录不与右侧的任何内容匹配时，右侧的列将全部匹配在结果集中显示为 NULL。我在查询中使用了 COALESCE 函数，如果前者为 NULL，它将有条件地将第一个参数替换为第二个参数。在这种情况下，如果 NULL，特价将被替换为零。我还将它与产品名称一起使用，以防在某些情况下缺少名称。