Question

I’m looking for some advice on how best to get the first record when using a join with multiple tables, as demonstrated below.

I have three tables:

• Leads <– this should be unique in the results
• LeadAddresses (joining
  table)
• Addresses

Normally I’d join them like this:

from t2
    in db.Leads
    .Where(o => t1.LeadId == o.Lead_ID)
from t4
    in db.LeadAddresses
    .Where(o => t2.Lead_ID == o.Lead_ID)
    .DefaultIfEmpty()
from t5
    in db.Addresses
    .Where(o => t4.Address_ID == o.Address_ID)
    .DefaultIfEmpty()

(if this is bad practice, let me know 😉

I’m looking to get a property from the Addresses table (the one with, say, the maximum ID) for each Lead record and project to a model:

select new LeadGridModel
        {
            ...
        });

Example:

Lead Company | City    | ZIP
==============================
Company 1    | Boston  | 00000
Company 2    | Houston | 00001

Answer 1

from l in db.Leads
from a in l.LeadAddresses.Select(la => la.Address).OrderByDescending(a => a.ID).Take(1).DefaultIfEmpty()
select new { l, a }

This might look tricky, but you understand it part by part:

1. Using OrderByDescending in combination with Take(1) we take the address with the maximum ID
2. Using DefaultIfEmpty we create a left-join.

Be aware that this pattern forces a loop-join due to limitation of SQL Server. For small result sets this is usually not a problem.