database - What are the differences between a superkey and a candidate key?

Question

What are the differences between a super key and a candidate key? I have already referred to wiki,dotnet spider and also Database Concepts 4th edition book. But I am unable to understand the concept. Can anyone please explain it with proper example?

Answer 1

Candidate key is a super key from which you cannot remove any fields.

For instance, a software release can be identified either by major/minor version, or by the build date (we assume nightly builds).

Storing date in three fields is not a good idea of course, but let's pretend it is for demonstration purposes:

year  month date  major  minor
2008  01    13     0      1
2008  04    23     0      2
2009  11    05     1      0
2010  04    05     1      1

So (year, major, minor) or (year, month, date, major) are super keys (since they are unique) but not candidate keys, since you can remove year or major and the remaining set of columns will still be a super key.

(year, month, date) and (major, minor) are candidate keys, since you cannot remove any of the fields from them without breaking uniqueness.