747b0392__5_1___Postgres_OnLine_Journal__Microsoft_SQL_Server_2008
[Feature] Can define custom aggregate functions [MySQL 5.1] Yes but only in C as UDF [PostgreSQL 8.4] Yes - any PL language and built-in C, SQL, PLPgSQL. [Microsoft SQL Server 2008] 
Yes - any .NET language, but not TRANSACT SQL. Why is Transact-SQL thrown out to dust like this?
Cross Compare of PostgreSQL 8.4, SQL Server 2008, MySQL 5.1 - Postgres OnLine Journal
Microsoft SQL Server 2008