Relational Model:

• Data is represented as a relation (table).
• A relation is a set of tuple.
• A relation is a table made up by columns (attributes, S₁,S₂,S₃,......S_n) and rows (tuples).
• It is a subset of Cartesian products.  (R is a subset of S₁ × S₂ × S₃ × ........ × S_n)
• Set operations (intersection, union, complement, difference) have to be performed.
• Each tuple is unique.  Every two tuples must be different.
• Because any two tuples has to be distinct,  there must be a key.
• Relation: made up of 2 parts:

Instance : a table, with rows and columns.
Schema : specifies name of relation, plus name and type of each column.
               e.g. Students(sid: string, name: string, login: string, age: integer, gpa: real)

A set of fields is a candidate key for a relation if :

1. No two distinct tuples (records) can have same values in all key fields, and
2. This is not true for any subset of the candidate key.
              Candidate key is a minimal (not minimum) of attributes to identify a tuple.
(Note:  Minimal means that if you remove any of them, the unique property will not be held (i.e. it will not be a key.))

Super key:

If the condition of part 2 in the above definition is false, it is a super key.

If there's more than one candidate key for a relation, one of the candidate keys is chosen (by DBA) to be the primary key.
e.g., sid is a key for Students.  (What about name?)  The set {sid, gpa} is a super key.
Primary and Candidate Keys in SQL
Possibly many candidate keys  (specified using UNIQUE), one of which is chosen as the primary key.

Most of the time, the primary key is a single field, but in very special circumstands, two or more fields can share the job.  This type of key is called a multifield key or compound key.
In Microsoft Access, hold Shift + click more than one row in the design view table, then ask KEY command.

Foreign key:

Set of fields in one relation that is used to 'refer’ to a tuple in another relation.
(Must correspond to primary key of the second relation.)
Like a 'logical pointer’.
If all foreign key constraints are enforced,  referential integrity is achieved, i.e., no dangling references.