Termonology
Bag
A bag is a group of tuples in no specific order.
Candidate Key
A candidate key is a minimal superkey for a relation. In other words, it is a set of attributes so that:
- The relation does not contian two distinct tuples with the same values in these attributes,
- There is no subset of these attributes which also satisfies (1).
Contradiction
A logic expression that always evaluates to false
NULL
A NULL basically means no or unknown value. Of course, two unknown values cannot be compared, so two NULLs cannot be compared either.
Primary Key
A Primary key is a unique key without support for NULL values
Set
A set is a group of distinct tuples in no specific order.
Superkey
A superkey is a set of attributes so that the relation does not contain two distinct tuples with the same values in these attributes.
Tautology
A logic expression that always evaluates to true
Tuple
A tuple is a ordered set of values. In a relational database a tuple is often called a row.
Unique Key
A Unique key is a set of attributes so that the relation does not contain two distinct tuples with the same values in the attributes if NULL values are not used. NULL values are incomparable, and thus considered different.
