查询，返回字符串，即使SELECT语句为空，也是错误的

Question

简单的问题，但不知道如何解决。 如果 SELECT 部分不返回任何内容，则以下查询应返回“无值”。我会收到错误消息：

[21000][1241] (conn=80188403) Operand should contain 1 column(s)

这个查询有什么问题？

SET @produktNummer = 01000051311;
SET @languageID = '2fbb5fe2e29a4d70aa5854ce7ce3e20b';
SELECT IFNULL(
      (SELECT product.id, product_number, property_group_translation.name AS AttributBez
       FROM product
       LEFT JOIN product_property 
         ON product.id = product_property.product_id
       LEFT JOIN property_group_option 
         ON product_property.property_group_option_id = property_group_option.id
       LEFT JOIN property_group_translation 
         ON property_group_option.property_group_id = property_group_translation.property_group_id
       WHERE product_number = @produktNummer
          AND property_group_translation.language_id = UNHEX(@languageID)
          AND (
              property_group_translation.name = '' OR
              property_group_translation.name IS NULL
              )
       )
  ,'No values');

如果我尝试不使用 ISNULL()，我将不会得到任何结果。如果我像这样改变查询，我会得到多条记录

SET @produktNummer = 01000051311;
SET @languageID = '2fbb5fe2e29a4d70aa5854ce7ce3e20b';
SELECT product.id, product_number, property_group_translation.name AS AttributBez
FROM product
LEFT JOIN product_property
ON product.id = product_property.product_id
LEFT JOIN property_group_option
ON product_property.property_group_option_id = property_group_option.id
LEFT JOIN property_group_translation
ON property_group_option.property_group_id = property_group_translation.property_group_id
WHERE product_number = @produktNummer
AND property_group_translation.language_id = UNHEX(@languageID)

Answer 1

您可以获得至少 1 行包含数据或“NoValue”的行。
包含其他侧参数作为子查询并将其他数据加入其中。

(
  select @produktNummer as produktNummer, UNHEX(@languageID) as languageID
  ) request

参见示例

SET @produktNummer = 01000051311;
SET @languageID = '2fbb5fe2e29a4d70aa5854ce7ce3e20b';
SELECT produktNummer,languageID
  ,coalesce(product.id,'NoValue') id
  , coalesce(product_number,'NoValue')product_number
  , coalesce(property_group_translation.name,'NoValue') AS AttributBez
FROM (
  select @produktNummer as produktNummer, UNHEX(@languageID) as languageID
  ) request
left join product p on p.product_number = request.produktNummer 
LEFT JOIN product_property
   ON product.id = product_property.product_id
LEFT JOIN property_group_option
   ON product_property.property_group_option_id = property_group_option.id
LEFT JOIN property_group_translation
   ON property_group_option.property_group_id = property_group_translation.property_group_id
WHERE property_group_translation.language_id = request.languageID