SQL Server - 当条件可能为空时如何进行内部联接？

Question

I am trying to get the Country from my factories AddressID directly in one Query with an Inner join. My problem is that the AddressID could be an empty string.

How do i check that and do some kind of exception or something similar? I if address ID is empty i need an empty string as result for f.Country.

This is my query for now, but if AddressID = '' then the row is not showing up in my result.

SELECT o.FactoryID,o.Name,o.Rating,o.ProductCategory,o.Emissions,o.LatestChanges,o.AddressID,o.ProductTags,f.Country 
FROM FactoryHistory o
INNER JOIN AddressHistory f on f.AddressID = o.AddressID
WHERE NOT o.LatestChanges = 'Deleted' AND o.IsCurrent = 1 AND f.IsCurrent = 1

I something in my question is missing or unclear, just let me know!

Answer 1

Just left join :

SELECT o.FactoryID, o.Name, o.Rating, o.ProductCategory, o.Emissions, o.LatestChanges, o.AddressID, o.ProductTags, f.Country 
FROM FactoryHistory o
LEFT JOIN AddressHistory f on f.AddressID = o.AddressID AND f.IsCurrent = 1
WHERE NOT o.LatestChanges = 'Deleted' AND o.IsCurrent = 1

If you still want to filter out factories whose AddressID is not null (assuming that by empty you mean null ) and that do not exist in the address table, than you can add a condition in the WHERE clause:

WHERE 
    NOT o.LatestChanges = 'Deleted' 
    AND o.IsCurrent = 1 
    AND (o.AddressID IS NULL OR f.AddressID IS NOT NULL)

It might be clearer with a negation:

WHERE 
    NOT o.LatestChanges = 'Deleted' 
    AND o.IsCurrent = 1 
    AND NOT (o.AddressID IS NOT NULL AND f.AddressID IS NULL)