Fancy title to get things going.
This is one of those things that'll be second nature once I start using it in anger, but until I do that, I'll consistently forget it.
C# 2.0 introduces nullable types, and the null coalescing operator (??) is a binary operator that works pretty much the same way as T-SQL's ISNULL() function.
So for:
a ?? b
think:
ISNULL(a, b)
One small caveat here is that b could of course be a nullable type as well, in which case the result of a ?? b is still a nullable type. If it's not, the result isn't either. See example below (ripped from C# 2.0 spec).
int? x = GetNullableInt();
int? y = GetNullableInt();
int? z = x ?? y;
int i = z ?? -1;
3 comments:
This probably makes more sense to people who know what SQL's ISNULL does -- I didn't, and it's not intuitive.
My understanding is that C#'s:
i = x ?? y;
.. is equivalent to ..
i = x.HasValue ? x : y ;
or for the ternary operator unaware
if(x.HasValue) { i = x } else { i = y }
Can you chain these? It feels as if you should be able to:
i = x ?? y ?? z ;
You can indeed chain them. ?? is right-associative, so your example of
i = x ?? y ?? z;
equates to
i = x ?? (y ?? z);
Conceptually, if you have a long chain of these, you'll get the value of the first non-null one, or null if they're all null.
Actually ISNULL() is chainable (at least on SQL Server 2005), so long as you use it in the same fashion as you would with the C# code, i.e. by repeating the "operator" (in this case ISNULL()).
So
ISNULL(a, ISNULL(b, c))
is equivalent (and I hope I've got my ordering right here) to
a ?? b ?? c
However I fully agree that in the SQL case, COALESCE(a, b, c) is much nicer.
In deference to the MS-only side of things, I have changed the original post to read T-SQL instead :)
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