FUNCTIONAL DEPENDENCY, KEYS & Closures- Finding Keys in various Examples in DBMS

[@firibuanyass]

Best explanation ever

[@mustansirbhagat]

in definition, it should be t1.x = t1.y right? because we are comparring 2 attributes in same tuple for each tuple

[@ritukapurclasses1591]

no, first you check if x is same for any two rows abd if it is, then we check for y. So, it’s the rule is t1.x = t2.x.

[@akhilreddy4852]

what about other keys??

[@subhadippatra7930]

The attribute closure which consists all the attributes of the relation forms the candidate key. On the other hand, a candidate key is an attribute which can uniquely identify any record of a relation . May you give me the possible intuition, if from an attribute let's say A we can reach other attribute then we can ensure that A can uniquely identify any record in the relation ?

[@deepanshubansal8010]

Nice English

[@subhadippatra7930]

Madam, isn't is true that candidate key is always minimal ? Because, as E is candidate key and now E is present in RHS of any FD CD->E then from CD we can reach each and every attribute. That's true. But CD is not minimal. So, CD should be a super key. How can it be a candidate key ?

[@ritukapurclasses1591]

Minimal doesn't means minimal in length. We say a key is not minimal if its subset is also a candidate key. As no subset of CD is a key, CD is minimal