This is probably a silly question... please humor me: I'm a Java/JS developer learning about databases as I go.
Suppose I have a table with columns A-Z, and declare the primary key to be (A, B, C). I understand that that forces each row to have a unique A, B, C combination. But what does it mean for querying? Can I very quickly query again column A and/or B and/or C, or just the combination of the 3? What is the difference between primary key (A, B, C) and primary key (A, B) with a secondary index on C? Thanks!
