问题
What difference between Super and Candidate key in ERDB?
Thanks.
回答1:
A superkey is a set of columns that uniquely identifies a row. A Candidate key would be a MINIMAL set of columns that uniquely identifies a row. So essentially a Superkey is a Candidate key with extra unnecessary columns in it.
回答2:
candidate key is a minimal superkey
回答3:
- Candidate key = minimal key to identify a row
- Super key = at least as wide as a candidate key
For me, a super key would generally introduce ambiguities over a candidate key
回答4:
Let's keep it simple
SuperKey - A set of keys that uniquely defines a row.So out of all the attributes if even any single one is unique then all the subsets having that unique attribute falls under superkey.
Candidate Key - A superkey out of which no further subset can be derived which can identify the rows uniquely, Or we can simply say that it is the minimal superkey.
回答5:
In nutshell: CANDIDATE KEY is a minimal SUPER KEY.
Where Super key is the combination of columns(or attributes) that uniquely identify any record(or tuple) in a relation(table) in RDBMS.
For instance, consider the following dependencies in a table having columns A, B, C and D (Giving this table just for a quick example so not covering all dependencies that R could have).
Attribute set (Determinant)---Can Identify--->(Dependent)
A-----> AD
B-----> ABCD
C-----> CD
AC----->ACD
AB----->ABCD
ABC----->ABCD
BCD----->ABCD
Now, B, AB, ABC, BCD identifies all columns so those four qualify for the super key.
But, B⊂AB; B⊂ABC; B⊂BCD hence AB, ABC, and BCD disqualified for CANDIDATE KEY as their subsets could identify the relation, so they aren't minimal and hence only B is the candidate key, not the others.
来源：https://stackoverflow.com/questions/3087654/superkey-vs-candidate-key