1 votes 1 votes An instance of a relational schema R(A,B,C,D) has distinct values for attribute A then A can be 1) candidate key 2)non key 3)super key 4)none Databases databases candidate-key + – Shivani gaikawad asked Sep 14, 2018 Shivani gaikawad 507 views answer comment Share Follow See all 0 reply Please log in or register to add a comment.
0 votes 0 votes Any attribute that can be used to uniquely identify a row can be a super key and a minimal super key is called as candidate key. Since the key here has only one attribute, it is a candidate key. goxul answered Sep 14, 2018 goxul comment Share Follow See all 6 Comments See all 6 6 Comments reply Shaik Masthan commented Sep 14, 2018 reply Follow Share why not it can be NON-KEY? think at the instance it is giving different values, may be after some time ( i mean at some other instance ), it have duplicates. 1 votes 1 votes arvin commented Sep 14, 2018 reply Follow Share but they have asked for that particular instance .. which should be candidate key only... 0 votes 0 votes goxul commented Sep 14, 2018 reply Follow Share I think for the given particular instance, A can be used as a candidate key. Of course, it might refer to duplicates later, but we don't know about that, so we can't comment on that. At least that's how I interpreted this question. 0 votes 0 votes Shaik Masthan commented Sep 14, 2018 reply Follow Share i too notice hat, But keys and Functional Dependencies can't be said to Particular instances. By knowing Keys and Functional Dependencies, we have to take instances. 0 votes 0 votes manisha11 commented Sep 14, 2018 reply Follow Share for a given instance, it should be a foreign key. 0 votes 0 votes Prince Sindhiya commented Sep 15, 2018 reply Follow Share @shaikh sir you are right 0 votes 0 votes Please log in or register to add a comment.
0 votes 0 votes Here ,we may say that A->BCD ,FD holds for this instance And $A^+={A,B,C,D}$ But FD s are defined on the schema itself not the instance, so based on the state of the instance we cannot say what holds for schema (there can be a many instances for R). Prince Sindhiya answered Sep 15, 2018 Prince Sindhiya comment Share Follow See all 0 reply Please log in or register to add a comment.