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Unit 4 (Entities (Entity Identifier (Relationships between entities:
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Unit 4
Entities
E.g - Customer , Guide or product
Other entities that could be considered include customer order, subject
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Entity Identifier
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Composite Primary Key - Sometimes two or even more attributes are needed to uniquely define a record
Secondary Key - If searches often need to be made on title or subject either or both of these fields could be defined as a secondary key
Relationships between entities:
3 possible relations:
One - to - One
One - to - many
Many - to - Many
Many - to - Many relationships are linked between tables , they cannot be directly linked
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Normlisation
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There are three levels of normal form you to know about: first, second and third normal form.
Flat File Database
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Suitable:
When storing only limited amounts of database
When not familiar with DBMS
Security is not a big concern
Database Design
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Referential Integrity
Referential integrity means no foreign key in one table can reference a non-existent record in related table
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New Table in SQL:
CREATE TABLE tblProduct
(Product CHAR(4) NOT NULL PRIMARY KEY
Description VARCHAR(20) NOT NULL,
Price CURRENCY)
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