LINQ - filter duplicates with condition

Question

I'm developing an .NET application where I need to filter data from SQL Server 2008 depending on a condition. My data structure looks like this: The result should be grouped and sorted by Key. By default, the row with Revision null should be returned for each Key, but if revisionParameter is set the row with the corresponding Revision number should be returned. If the parameter is two, the output should be: ``` abc 2 FALSE def null TRUE ghj 2 FALSE klm null TRUE ``` How can I accomplish this in LINQ? Thanks EDIT: Response to lazyberezovsky: Your LINQ expression rewritten as lambda: ``` partial void RuleEntriesByUserSaveId_PreprocessQuery(int? UserSaveId, ref IQueryable<RuleEntry> query) { query = query.Where(re => re.Revision == null || re.Revision == value) .GroupBy(re => re.Key) .Select(g => g.FirstOrDefault(x => x.Revision != null) ?? g.First()); } ```

Accepted Answer

```
from r in Table
where !r.Revision.HasValue || r.Revision.Value == value
group r by r.Key into g
select g.FirstOrDefault(x => x.Revision.HasValue) ?? g.First()
```

That will select either records which have revision equal to `null` or to specified value. After grouping by key, we try to find any record which will have revision with value (that value will be equal to filter). If there is no records with revision, we just take first record from group.

LINQ - filter duplicates with condition

Solution

Problem