Superkey vs. Candidate key

What difference between Supe开发者_C百科r and Candidate key in ERDB?

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.

candidate key is a minimal superkey

• 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

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.

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.