Question

运行以下查询时，我遇到了丢失数据的问题。有些产品有特殊价格，存储在表oc_product_special中，有些产品的常规价格存储在表oc_product中。我发现如果表oc_product_special中有特殊价格，它只显示数据。它省略了没有特价和正常价格的任何数据。我不知道如何解决这个问题。如何以及在哪里可以添加条件语句或类似

如果有正常价格，没有特价则显示正常价格，0表示特价。




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

Answer 1

使用LEFT JOIN，即使它们与右侧的任何内容不匹配，也会在联接的左侧保留记录：

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

<强>解释

在LEFT JOIN中，当联接左侧的记录不与右侧的任何内容匹配时，右侧的列将全部显示为{{ 1}}在结果集中。我在查询中使用了NULL函数，如果前者为COALESCE，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，特殊价格将被替换为零。我也将它与产品名称一起使用，以防在某些情况下缺少名称。

Answer 2

在oc_product_special上使用左连接

    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
        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 ASC

在你的情况下匹配值的内连接si不匹配所以..use left join

在mysql查询中缺少数据，需要在内连接中使用条件语句

2 个答案: