How do you create a composite foreign key when key being referenced is also a foreign key If you have table a (column x) which is 1 to many with table b (columns x, y) which is 1 to many with table c (columns y, z), it's pretty straight forward to define the foreign keys.

If for performance reasons, table c was altered to be (columns x, y, z) is there a way to specify that table c (columns x, y) have a foreign key relationship to table b's (columns x, y)?  

Previously table c contained 2 single column foreign keys x --> table a and y --> table b.  While this is correct, it creates erroneous results when trying to use commercial tools to create database diagrams.
You can create a composite foreign key from B to C:

alter table TableC
     add constraint FK_TableC_XY
     foreign key (x, y)
     references TableB (x, y)


That's better than two foreign keys: this will ensure that the combination of (x, y) is present, and not just the individual values.