Conditional Inner Join

Question

I want to be able to inner join two tables based on the result of an expression.

What I\'ve been trying so far:

INNER JOIN CASE WHEN Reg         


        

Answer 1

So I had a scenario where there were three email columns in one table (don't ask why) and any of them could be null (or empty). In this example code I will just deal with a case where it is null.

I had to join it to another table by any of the emails to retrieve the users firstname.

Here is what worked

select  

m.email1,
m.email2,
m.email3, 
m2.firstName

from MyTable m

left join MyOtherTable m2 on m2.Email = 
case when m.email1 is null then 

    case when m.email2 is null then 

        case when m.email3 null then 
            'nonexistent@mydomain.com' -- i stopped here
        else m.email3 end

    else wm.email2 end

else m.email1 end

Obviously you would include further conditions like

case when m.email1 is null or m.email1 = '' then ...

To cover for empty values.