Get distinct results from database that has a "dictionary" - sql

Question

I have a database with a "dictionary" like this:
Id (INT) | Key (VARCHAR) | Value (VALUE) | ForeignKey (INT?)
------------------------------------------------------------
1 | foo | bar | 23
2 | bar | foo | NULL
3 | foobar | value | NULL
4 | foobar | othervalue | 47
Now I want to get all keys and values for a specific foreign key, but also all foreign keys that are NULL, so that the result looks like this:
ForeignKey = 23: ForeignKey = 47:
foo | bar bar | foo
bar | foo foobar | othervalue
foobar | value
Originally I tried this approach:
dbContext.Table
.Where(t => t.ForeignKey == 47 || t.ForeignKey == null)
but that gives me foobar twice.
I then though about multiple requests and unionizing the results:
var t1 = _dbContext.Table
.Where(t => t.ForeignKey == 47);
var t2 = _dbContext.Table
.Where(t => t.ForeignKey == null && !t1.Any(tt => tt.Key == t.Key));
var final = t1.Union(t2);
This seemingly works, but it creates this SQL (roughly) which seems excessive with three SELECT, three WHERE and a UNION
SELECT [t1].[Id], [t1].[Key], [t1].[Value], [t1].ForeignKey
FROM [Table] AS [t1]
WHERE [t1].[ForeignKey] = 47
UNION
SELECT [t2].[Id], [t2].[Key], [t2].[Value], [t2].ForeignKey
FROM [Table] AS [t2]
WHERE [t2].[ForeignKey] IS NULL AND NOT (EXISTS (
SELECT 1
FROM [Table] AS [t3]
WHERE ([t3].[ForeignKey] = 47) AND ([t3].[Key] = [t2].[Key])))
I just have a gut feeling that "there's got to be a better way"... so, is there? How can I get keys and values for a specific foreign key, but also keys not already fetched that are NULL?

Answer 1

The SQL you're seeing doesn't seem excessive to me. Unless you've tested this and found it to have unacceptably bad performance, I wouldn't necessarily spend too much time trying to optimize it.
Here's an alternative approach, though, which may be preferable. You'd have to do some testing to see if it yields better SQL, execution plan, and performance. Mostly I'd go with whichever approach will be easier for other developers to understand.
var final = dbContext.Table.Where(t => t.ForeignKey == 47 || t.ForeignKey == null)
.GroupBy(t => t.Key)
.Select(g => g.OrderBy(t => t.ForeignKey == null ? 1 : 0).FirstOrDefault());