Linq outer join using inequality?

问题

In SQL I'd say:

select a.*
from TableA a 
left join TableB b on a.Type = b.Type and a.SomeDate < b.AnotherDate
where b.ID is null

This would select all records in TableA where no record exists in TableB of the same Type and later date.

In Linq, how do you do this?

from a in TableA
join b in TableB on a.Type equals b.Type into j // what about the comparator?
from x in j.DefaultIfEmpty()
where x == null
select a;

Thanks!

EDIT:

A few good answers have been proposed, all of which address the specific need expressed in this question, but they're all basically workarounds. They all translate to a nested "exists" queries in one way or another, whereas the SQL in the question is one neat query without any nesting. The case given here is just an example of a general principle; what I'd really like to see is a Linq expression that will translate to (roughly) the syntax of the above SQL query.

回答1:

var results = TableA.Where(a => 
                 !TableB.Any(b => a.Type == b.Type && a.Date < b.Date))

If you want the linq query to be exactly as your SQL you can write:

var result = from a in TableA
             from b in TableB.Where(b => a.Type = b.Type && a.SomeDate < b.AnotherDate).DefaultIfEmpty()
             where b == null
             select a;

But I would say that the first solution is better as the where b == null would result in a filter operation in the queryplan.

回答2:

Something like this ought to help:

var results = 
    (from itemA in TableA
    from itemB in TableB
    where itemA.Type != itemB.Type && itemA.Date < itemB.Date
    select itemA).Distinct();

回答3:

from a in tableA
let rights =
  from b in tableB
  where a.Type == b.Type && a.Date < b.Date
  select b
where !rights.Any()
select a;

It's translated into:

SELECT [t0].[Type] AS [Type], [t0].[SomeDate] AS [SomeDate]
FROM [TableA] AS [t0]
WHERE NOT (EXISTS(
    SELECT NULL AS [EMPTY]
    FROM [TableB] AS [t1]
    WHERE ([t0].[Type] = [t1].[Type]) AND ([t0].[SomeDate] < [t1].[AnotherDate])))

来源：https://stackoverflow.com/questions/6403626/linq-outer-join-using-inequality

标签

linq

linq-to-sql