[英]How to convert SQL subquery to Joins
如何将SQL子查询转换为Joins:
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]);
我认为编写查询的最佳方法是:
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JOIN
编写查询,但我认为这更容易理解,并且当比较表具有NULL
值时,它将按预期工作。
尝试这个;
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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