如何将SQL子查询转换为联接

如何将SQL子查询转换为联接：

SELECT [COUNTRY] FROM [CountryMappingDB].[dbo].[DNB_Country_mapping_poc.ss] 
WHERE [COUNTRY] NOT IN 
(SELECT [EntityAttributeValue]  FROM [CountryMappingDB].[dbo].[DNB_MappingData.ss]
UNION ALL
SELECT [SalesCountryName] FROM [CountryMappingDB].[dbo].[DNB_Country_mapping_poc_dimSalesCountry.ss]);

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我认为编写查询的最佳方式是：

SELECT [COUNTRY]
FROM [CountryMappingDB].[dbo].[DNB_Country_mapping_poc.ss]  ss
WHERE NOT EXISTS (SELECT 1
                  FROM [CountryMappingDB].[dbo].[DNB_MappingData.ss] ss2
                  WHERE ss2.County = ss.Country
                 ) AND
      NOT EXISTS (SELECT 1
                  FROM [CountryMappingDB].[dbo].[DNB_Country_mapping_poc_dimSalesCountry.ss] ss3
                  WHERE ss3.SalesCountryName = ss.Country
                 );

尽管您可以使用联接编写查询，但我认为这更容易理解，并且当比较表具有空值时，它将按预期工作。

试试这个

SELECT [COUNTRY] FROM [CountryMappingDB].[dbo].[DNB_Country_mapping_poc.ss] 
left join
    (SELECT MyData=[EntityAttributeValue]  FROM [CountryMappingDB].[dbo].[DNB_MappingData.ss]
    UNION ALL
    SELECT MyData=[SalesCountryName] FROM [CountryMappingDB].[dbo].[DNB_Country_mapping_poc_dimSalesCountry.ss]
    ) countries on [COUNTRY]=countries.MyData
WHERE countries.MyData is null

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为什么你觉得有必要这样做？子查询没有什么错