Which one is correct w.r.t. RDBMS ?
primary key ⊆ candidate key ⊆ super key
In the relational model a super key is any set of attributes that uniquely identifies every tuple of a relation. A candidate key is a minimal super key (no proper subset of it is a super key). The primary key is the one candidate key chosen by the designer to identify tuples.
Viewing each kind of key as a set of keys of the relation: every primary key is a candidate key, and every candidate key is a super key, while the converse inclusions do not hold (a super key may carry extra attributes, and a relation may have several candidate keys of which only one is primary). This gives the chain primary key ⊆ candidate key ⊆ super key.
Placing super key inside candidate key would make every super key minimal, which is false. Placing candidate key or super key inside primary key would mean every candidate key is the primary key, which fails whenever a relation has more than one candidate key. Putting super key between primary key and candidate key reverses the candidate/super relationship.
primary key ⊆ candidate key ⊆ super key option is correct.
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