Many people will suggest you use MERGE, but I caution you against it. By default, it doesn't protect you from concurrency and race conditions any more than multiple statements, but it does introduce other dangers:
http://www.mssqltips.com/sqlservertip/3074/use-caution-with-sql-servers-merge-statement/
Even with this "simpler" syntax available, I still prefer this approach (error handling omitted for brevity):
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION;
UPDATE dbo.table SET ... WHERE PK = @PK;
IF @@ROWCOUNT = 0
BEGIN
INSERT dbo.table(PK, ...) SELECT @PK, ...;
END
COMMIT TRANSACTION;
A lot of folks will suggest this way:
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION;
IF EXISTS (SELECT 1 FROM dbo.table WHERE PK = @PK)
BEGIN
UPDATE ...
END
ELSE
BEGIN
INSERT ...
END
COMMIT TRANSACTION;
But all this accomplishes is ensuring you may need to read the table twice to locate the row(s) to be updated. In the first sample, you will only ever need to locate the row(s) once. (In both cases, if no rows are found from the initial read, an insert occurs.)
Others will suggest this way:
BEGIN TRY
INSERT ...
END TRY
BEGIN CATCH
IF ERROR_NUMBER() = 2627
UPDATE ...
END CATCH
However, this is problematic if for no other reason than letting SQL Server catch exceptions that you could have prevented in the first place is much more expensive, except in the rare scenario where almost every insert fails. I prove as much here:
Not sure what you think you gain by having a single statement; I don't think you gain anything. MERGE is a single statement but it still has to really perform multiple operations anyway - even though it makes you think it doesn't.