How to achieve Left Excluding JOIN using LINQ?

Question

How to achieve Left Excluding JOIN using LINQ?

In SQL:

SELECT  
FROM Table_A A
LEFT JOIN Table_B B
ON A.Key = B.Key
WHERE B.Key I         


        

Answer 1

You need DefaultIfEmpty() for the LEFT JOIN, then you can check if the joined value is null:

var result = from a in Table_A
             join b in Table_B on a.Key equals b.Key into j
             from b in j.DefaultIfEmpty()
             where b == null
             select new { ... };

Answer 2

An even faster way

var result = from a in Table_A
    where !Table_B.Select(b => b.Key).Contains(a.Key)
    select new { ... };

Answer 3

Easier would be to write like this:

var result = from a in Table_A
             where !Table_B.Any(b => b.Key == a.key)
             select new { ... };