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Participation Constraints

 

Key: subset of attributes that uniquely identifies an entity (candidate key)


Entity Schema:

The meta-information of entity type name, attributes (and associated domain), key constraints Entity Types tend to correspond to nouns; attributes are also nouns albeit descriptions of the parts of entities. May have null values for some entity attribute instances – no mapping to domain for those instances.
 

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Many to Many
 

Keys : Keys are very important part of Relational database. They are used to establish and identify relation between tables. They also ensure that each record within a table can be uniquely identified by the combination of one or more fields within a table.


Super Key:
 It is a collection of one or more attributes used to uniquely identify an entity in entity set.


Candidate key:
 It is a minimal super key of which no proper subset is also a super key.


Primary key:
 It is the candidate key which is used to uniquel identify an entity in entity set.


Composite key:
 Key that consists of two or more attributes that uniquely identity an entity occurance is called Composite key. But any attribute that makes up the Composite key is not a simple key in its own.

Foreign key: term used in relational databases (but not in the E-R model) for an attribute that is the primary key of another table and is used to establish a relationship with that table where it appears as an attribute also.

So a foreign key value occurs in the table and again in the other table. This conflicts with the idea that a value is stored only once; the idea that a fact is stored once is not undermined.


Secondary or Alternative key :
 The candidate key which are not selected for primary key are known as secondary keys or alternative keys.


Non-key attribute :
 Non-key attributes are attributes other than candidate key attributes in a table

Non-prime attribute : Non-prime attributes are attrubutes other than primary attribute.

Weak entity types: Entity types that do not have primary key attribute are called weak entity type.






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