对于多个选择SQL中的某些值，返回NULL

Question

我正在使用PHP开发一个网站并拥有这段代码：

INSERT INTO items (producttype, firstcode, secondcode, brand, retailer, firstsize, secondsize)
          SELECT ?, productcodes.id, productcodes.id, brands.id, retailers.id, sizes.id, sizes.id
          FROM productcodes, brands, retailers, sizes
          WHERE LOWER(productcodes.code)=LOWER(?)
          AND LOWER(productcodes.code)=LOWER(?)
          AND LOWER(brands.name)=LOWER(?)
          AND LOWER(retailers.name)=LOWER(?)
          AND LOWER(sizes.id)=LOWER(?)
          AND LOWER(sizes.id)=LOWER(?)

但是当我有这样的输入时

INSERT INTO items (producttype, firstcode, secondcode, brand, retailer, firstsize, secondsize)
          SELECT 3, productcodes.id, productcodes.id, brands.id, retailers.id, sizes.id, sizes.id
          FROM productcodes, brands, retailers, sizes
          WHERE LOWER(productcodes.code)=LOWER(NULL)
          AND LOWER(productcodes.code)=LOWER(NULL)
          AND LOWER(brands.name)=LOWER('Canada')
          AND LOWER(retailers.name)=LOWER('WE')
          AND LOWER(sizes.id)=LOWER('XL')
          AND LOWER(sizes.id)=LOWER(NULL)

尽管大多数列都可以为null（这在代码的早期检查过。如果只有一列实际返回0行，它将为所有列返回null（空）。

Answer 1

感谢您提供的所有重要提示/建议/解释。

我已经设法通过使用此查询来使我的代码工作，它可能是一个巨大的黑客或服务器真的很重，但现在它的工作原理。如果有人有任何改进此代码的建议，请留言。

INSERT INTO items 
        (producttype, 
         firstcode, 
         secondcode, 
         brand, 
         retailer, 
         firstsize, 
         secondsize)
SELECT 3, 
   IFNULL((SELECT productcodes.id 
           FROM   productcodes 
           WHERE  LOWER(productcodes.code) = LOWER(NULL)), NULL), 
   IFNULL((SELECT productcodes.id 
           FROM   productcodes 
           WHERE  LOWER(productcodes.code) = LOWER(NULL)), NULL), 
   IFNULL((SELECT brands.id 
           FROM   brands 
           WHERE  LOWER(brands.name) = LOWER('Canada')), NULL), 
   IFNULL((SELECT retailers.id 
           FROM   retailers 
           WHERE  LOWER(retailers.name) = LOWER('WE')), NULL), 
   IFNULL((SELECT sizes.id 
           FROM   sizes 
           WHERE  LOWER(sizes.size) = LOWER('XL')), NULL), 
   IFNULL((SELECT sizes.id 
           FROM   sizes 
           WHERE  LOWER(sizes.size) = LOWER(NULL)), NULL)