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Definition of the Primary key,Foreign Key,Null and Domain.

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Primary Key

A primary key is one or more columns in a table whose values are used to uniquely identify each of the rows in that table.

Foreign Key

A Foreign Key is a column or group of columns that draws its value from a domain of the primary key in a related table. And/or a FK is the means of connecting tables in a database and some explanation of how it does this.

Example:-List attributes for the new entity ‘Film-Role’, clearly identifying the primary and foreign keys.

The attributes for the line-item entity would be:

  • FilmRoleNo (PK)-accept unique name
  • ActorID (FK)
  • FilmID (FK)

OR

ActorID, FilmID as a concatenated PK with both attributes also being FKs.

Domain

A domain is a pool of valid values from which the actual values of a column are drawn.

Null

Indication that a value is unknown

briefly describe a relation as used in the relational data model.

A relation is a table that obeys a specific set of rules.

Describe the rules that a relation must obey in the context of the relational data model.

  • Every relation in a single database must have a distinct name.
  • Every column in a relation must have a distinct name in that relation.
  • All entries in a column must be of the same type.
  • The ordering of columns has no significance.
  • Duplicate rows are not allowed.
  • The ordering of rows has no significance.
  • Each cell can only contain an atomic value (no multiple values in a single cell).
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