Matching data from three tables in Sql Server 2008

Question

I have three tables Despatch, Activation and Replaced as below:

Answer 1

I think you could use left joins to do this. Try this query, with your sample data it produces the desired output, except for the ApprovedQty, but I don't get how you arrived at 12 for that with the sample data:

select 
    d.LOTQty, 
    ApprovedQty = count(d.ProductNo),
    d.DispatchDate,
    Installed = count(a.ProductNo) + count(r.ProductNo)
from 
    Despatch d 
left join 
    Activation a 
     on d.ProductNo = a.ProductNo 
    and d.DispatchDate < a.ActivationDate 
    and d.LOTQty = a.LOTQty
left join 
    Replaced r 
      on d.ProductNo = r.ProductNo 
     and d.DispatchDate < r.RecordDate
     -- only count Replaced when there is no match in Activation
     -- or DispatchDate is greater then ActivationDate
     and (a.ActivationDate is null or a.ActivationDate < d.DispatchDate)
where 
    d.LOTQty = 20
group by 
    d.LOTQty, d.DispatchDate

this would output:

LOTQty  ApprovedQty DispatchDate    Installed
20      6           2013-08-07      5