LINQ中的左外部联接不起作用

Question

I'm trying to do a left outer join in Linq but the below code is not working

 var result = from dataRows1 in agdt.AsEnumerable()
              join dataRows2 in hwt.AsEnumerable()
              on dataRows1.Field<string>("ID") equals dataRows2.Field<string>("HWID") 
              where ((dataRows2.Field<string>("HWID") == null) && 
                    (dataRows1.Field<string>("TYPE")=="a"))
              select dataRows1;

Without the where clauses I receive about 37000 rows and with it I receieve 0. The agdt table has 12000 rows and the hwt table has 6000. This is getting very frustrating. Can someone please help?

Answer 1

You are missing the DefaultIfEmpty method call.

From what I understand from your query, it should look something like:

var result = from dataRows1 in agdt.AsEnumerable()
          join dataRows2 in hwt.AsEnumerable()
          on dataRows1.Field<string>("ID") equals dataRows2.Field<string>("HWID") 
          into groupJoin
          from leftOuterJoinedTable in groupJoin.DefaultIfEmpty()
          where (leftOuterJoinedTable == null && 
                (dataRows1.Field<string>("TYPE")=="a"))
          select dataRows1;

Answer 2

It seems to me that this in essence would be the same as running the following SQL query

SELECT DR1.* FROM DataRows1 DR1 INNER JOIN DataRows2 DR2 ON DR1.ID=DR2.HWID WHERE DR2.HWID IS NULL AND DR1.Type='a'

Essentially your LINQ is doing an inner join and then executing the where. To truly do a left join, see the link

LEFT OUTER JOIN in LINQ