With a brief introduction to the topic of Database Design, we started with some explanation on very important terminology related to the subject of "Database Design - Normalization and Normal Forms" in the last post with the same heading. There are some more such terminologies to be understood before we actually take to actual design and normalization and hence we shall continue with the same here -
Mapping Cardinality :
Also called Cardinality Ratio express the number of entities to which another entity can be associated via a relationship set. For a binary relationship (relationship involving two participant entities i.e. relationship with degree 2. visit Part I for explanation.) set R between entity sets A & B, there may be one of the following mapping cardinality -
- Superkey - Set of one or more attributes, that taken collectively, allow us to identify an entity in an entity set. (Example empid, empname, empjob, entrypassnumber etc. taken collectively or together).
- Candidate Key - Superkey for which no proper subset (set containing any less attributes than in consideration) is a superkey is called as candidate key (For example empid or entrypassnumber are candidate keys in the above example).
- Primary Key - One of the chosen candidate keys to identify an entity uniquely in the entity set is called as Primary Key. The choice is driven mainly by the candidate key value of which may never be required to change and is never a null is more eligible to become the primary key. (For example "empid" may become primary key whereas "entrypassnumber" may not, though also unique but may change if new entry pass may be required to be issued in case of loss and also may temporarily have null value until the new one is issued).