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    Relational Model, ER Design and Schema Architecture Short Notes for GATE CS

    Relational Model, ER Design and Schema Architecture short notes for GATE CS: 3 study cards covering concepts, formulas, shortcuts and exam traps, plus solved

    relational model er design and schema architecture short notes

    Quick Revision: ER Modeling Essentials

    Quick Revision Checklist

    Symbols

    • Rect: Entity (Double = Weak)
    • Diamond: Relationship (Double = Identifying)
    • Oval: Attribute (Double = Multi-valued)
    • Double Line: Total Participation

    Weak Entity Rules

    • Total participation in identifying relationship
    • Identifying relationship is many-to-one
    • Complete ID = (Owner PK, Partial Key)

    Is it a Weak Entity?

    Cannot exist without owner?
    Cannot be uniquely identified without owner's key?
    Represents a component/part of owner?
    If all yes → Weak Entity
    Avoid These Mistakes
    • Calling every dependent entity "weak"
    • Forgetting total participation for weak entities
    • Using many-to-many for identifying relationships

    Quick Revision: Keys and Constraints

    Essential Facts

    Structure

    • Degree: Number of Attributes (Columns)
    • Cardinality: Number of Tuples (Rows)

    Keys

    • Super Key: Uniquely identifies tuple (may have redundancy).
    • Candidate Key: Minimal Super Key (no redundancy).
    • Primary Key: Chosen Candidate Key (Unique + Not Null).
    • Foreign Key: References PK of another table (Can be Null/Duplicate).

    Integrity Constraints

    1. Domain: Atomic, correct data type.
    2. Entity: PK NULL.
    3. Referential: FK matches PK or is NULL.
    4. Key: PK is Unique.
    Common Traps
    • FK must be Unique.
    • Only one FK per table.
    • FK cannot reference same table.
    • Degree = Number of Rows.

    Three-Schema Architecture: Quick Revision

    Three-Schema Architecture: Quick Revision

    Component Description Key Property
    External Schema User-specific views of the data. Hides the rest of the database.
    Logical Schema Global structure of the entire database. Defines relations, attributes, constraints.
    Physical Schema Actual storage details on disk. Defines files, indexes, compression.
    Physical Independence Modify physical without changing logical. Easy to achieve.
    Logical Independence Modify logical without changing external. Hard to achieve.
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    Relational Model, ER Design and Schema Architecture Short Notes for GATE CS

    Relational Model, ER Design and Schema Architecture short notes for GATE CS: 3 study cards covering concepts, formulas, shortcuts and exam traps, plus solved practice questions.

    Quick Revision: ER Modeling Essentials

    Quick Revision Checklist

    Symbols

    • Rect: Entity (Double = Weak)
    • Diamond: Relationship (Double = Identifying)
    • Oval: Attribute (Double = Multi-valued)
    • Double Line: Total Participation

    Weak Entity Rules

    • Total participation in identifying relationship
    • Identifying relationship is many-to-one
    • Complete ID = (Owner PK, Partial Key)

    Is it a Weak Entity?

    Cannot exist without owner?
    Cannot be uniquely identified without owner's key?
    Represents a component/part of owner?
    If all yes → Weak Entity
    Avoid These Mistakes
    • Calling every dependent entity "weak"
    • Forgetting total participation for weak entities
    • Using many-to-many for identifying relationships

    Quick Revision: Keys and Constraints

    Essential Facts

    Structure

    • Degree: Number of Attributes (Columns)
    • Cardinality: Number of Tuples (Rows)

    Keys

    • Super Key: Uniquely identifies tuple (may have redundancy).
    • Candidate Key: Minimal Super Key (no redundancy).
    • Primary Key: Chosen Candidate Key (Unique + Not Null).
    • Foreign Key: References PK of another table (Can be Null/Duplicate).

    Integrity Constraints

    1. Domain: Atomic, correct data type.
    2. Entity: PK NULL.
    3. Referential: FK matches PK or is NULL.
    4. Key: PK is Unique.
    Common Traps
    • FK must be Unique.
    • Only one FK per table.
    • FK cannot reference same table.
    • Degree = Number of Rows.

    Three-Schema Architecture: Quick Revision

    Three-Schema Architecture: Quick Revision

    Component Description Key Property
    External Schema User-specific views of the data. Hides the rest of the database.
    Logical Schema Global structure of the entire database. Defines relations, attributes, constraints.
    Physical Schema Actual storage details on disk. Defines files, indexes, compression.
    Physical Independence Modify physical without changing logical. Easy to achieve.
    Logical Independence Modify logical without changing external. Hard to achieve.

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