Linq to sql join with multiple "OR" filters

Question

I need to implement this in Linq-to-sql

SELECT DISTINCT c.*
FROM dbo.Collector_Capital_Equipment c
LEFT OUTER JOIN dbo.Lab_Space s ON c.Room = s.Entry_Bar_Code 
                                OR c.HomeRoom = s.Entry_Bar_Code
WHERE s.id = 1021645 

All the research I've done on this says that linq-to-sql can't support an "OR" multiple join, and the suggestions are to instead do two joins like this:

SELECT DISTINCT c.*
FROM dbo.Collector_Capital_Equipment c
LEFT OUTER JOIN dbo.Lab_Space s ON c.Room = s.Entry_Bar_Code 
LEFT OUTER JOIN dbo.Lab_Space s2 ON c.HomeRoom = s2.Entry_Bar_Code
WHERE s.id = 1021645 

Those aren't actually the same query though as they'll return different results. Short of just putting the raw SQL into my C# program at this point, is there any way to accomplish the above?

Answer 1

Let's start from beginning. If you use WHERE with column from OUTER JOIN table it means that your query:

SELECT DISTINCT c.*
FROM dbo.Collector_Capital_Equipment c
LEFT OUTER JOIN dbo.Lab_Space s ON c.Room = s.Entry_Bar_Code 
                                OR c.HomeRoom = s.Entry_Bar_Code
WHERE s.id = 1021645 

is logically equivalent to:

SELECT DISTINCT c.*
FROM dbo.Collector_Capital_Equipment c
JOIN dbo.Lab_Space s 
  ON c.Room = s.Entry_Bar_Code 
  OR c.HomeRoom = s.Entry_Bar_Code
WHERE s.id = 1021645;

And this could be achieved with CROSS JOIN (pseudocode):

var q = from c in Collector_Capital_Equipment
       from s in Lab_Space
       where s.id == 1021645 
          && (s.Entry_Bar_Code == c.Room || c.HomeRoom == s.Entry_Bar_Code)
       select ...

---

I assume that you really want to generate query:

SELECT DISTINCT c.*
FROM dbo.Collector_Capital_Equipment c
LEFT OUTER JOIN dbo.Lab_Space s 
  ON (c.Room = s.Entry_Bar_Code OR c.HomeRoom = s.Entry_Bar_Code)
  AND s.id = 1021645

which could be represented as:

SELECT c.*
FROM dbo.Collector_Capital_Equipment c
LEFT OUTER JOIN dbo.Lab_Space s 
  ON c.Room = s.Entry_Bar_Code AND s.id = 1021645
INTERSECT
SELECT c.*
FROM dbo.Collector_Capital_Equipment c
LEFT OUTER JOIN dbo.Lab_Space s 
  ON c.HomeRoom = s.Entry_Bar_Code AND s.id = 1021645

And above query could be achieved with LINQ using set operators.

db<>fiddle demo

Answer 2

I don't think I've ever seen someone use an OR clause on a join before. Does MS SQL Server even support that in SQL?

I'd probably just split this off into two queries, to be honest.

SELECT entry_bar_code 
FROM dbo.Lab_Space WHERE id = 1021645;

With the result from that being fed into

SELECT DISTINCT * 
FROM dbo.Collector_Capital_Equipment 
WHERE c.room == <barcode> OR c.homeRoom == <barcode>