Microsoft documentation says that U-SQL uses the explicit ANSI SQL join syntax. But there's no actual join syntax in SQL 92 or SQL 99 for SEMIJOINs, they have to be coded using something else (as it does in T-SQL). But the Microsoft U-SQL documentation is pretty sparse, it doesn't mention semiloins or antisemijoins. The best documentation I've found is on this website: Mike McQuillan's article on joins in U-SQL referenced in the explanation for today's QoTD, and even that is rather sparse on semijoins, although not as sparse as the Microsoft doc. I wonder if that stairway will bring more detail on U-SQL join syntax in a future article.