Daca lucrezi cu SQL 2008 te avantajeaza "MERGE". De exemplu:

MERGE INTO B AS trg
USING A AS src
ON src.id = trg.id
WHEN MATCHED THEN UPDATE SET trg.y = src.x
WHEN NOT MATCHED THEN INSERT (id, y) VALUES(src.id, src.x);

"MERGE" este inclusa in ANSI-SQL si te va avertiza in cazul unor probleme ca aceea mentionata de Razvan.