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Mapping ER to Relational Model

Mapping ER to Relational Model. Each strong entity set becomes a table. Each weak entity set also becomes a table by adding primary key of owner entity set. To create a table for an entity set: Non-composite, single-valued attributes become attributes of the table.

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Mapping ER to Relational Model

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  1. Mapping ER to Relational Model Each strong entity set becomes a table. Each weak entity set also becomes a table by adding primary key of owner entity set. To create a table for an entity set: • Non-composite, single-valued attributes become attributes of the table

  2. Mapping ER to Relational Model To create a table for an entity set: • Composite attributes: either make the composite a single attribute or use individual attributes for components, ignoring the composite • Multi-valued attributes: remove them to a new table along with the primary key of the original table (still keep the primary key in original table)

  3. Mapping ER to Relational Model • Binary Relationships: • 1:M- place the primary key of 1 side in table of M side as foreign key • 1:1- use the primary key of one of the two tables as foreign key in the other table • M:M- create a relationship table with primary keys of related entities, along with any relationship attributes

  4. Mapping ER to Relational Model • Ternary or higher degree relationships: construct relationship table composed of primary keys of the participating entity sets, along with any relationship attributes

  5. Keys in Relation Model • Relations (tables) never have duplicate records, so you can always tell records apart; implies there is always a key (which may be a composite of all attributes, in worst case) • Superkey: set of attributes that uniquely identifies records • Candidate key: a superkey such that no proper subset of itself is also a superkey (i.e. it has no unnecessary attributes) • Primary key: The candidate key chosen for unique identification of records • A foreign key is an attribute or combination of attributes that is the primary key of some relation (referred to as the home relation of the foreign key)

  6. Integrity Constraints • Integrity constraints are rules or restrictions that apply to all instances of the database • Types of constraints • Domain constraint - limits set of values for attribute • Entity integrity: no attribute of a primary key can have a null value • Referential integrity: each foreign key value must match the primary key value of some record in its home relation or be completely null. • General constraints or business rules: may be expressed as extra table constraints or assertions

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