SQL Server: Can you join implicitly using key relationships?
You always have to specify the columns you are joining on, regardless of any foreign key relationships.

Why? Because otherwise you couldn't do a full outer join if a foreign key existed between the tables, which would lead to identical queries producing different results for non-obvious reasons.

Also, what if you explicitly join on a different field? Would SQL behave as if you had included both fields in the join statement? Would you have to somehow tell it to ignore the foreign-key relationship for this one query?

FYI, the term "implicit join" in SQL actually refers to something entirely different, a SELECT statement made using a comma instead of a JOIN keyword:

 SELECT *
 FROM table_A, table_B
 WHERE table_A.key = table_B.key


vs an "Explicit" join:

 SELECT *
 FROM table_A JOIN table_B
 ON table_A.key = table_B.key


(This "old style" of implicit joins still works, but is not recommended.)