ISYS DAY 4

ISYS DAY 4 - can RELATIONSHIP at most how many instances of...

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ERD Fundamentals Models o ER Diagrams help understand the big picture. To understand what's going on o Entity: an object or concept about which we want to tore data Person (customer, supplier) Thing (order, item) Event Place o Test for entities More than one instance More than one attribute o Relationships Naturrally occuring associations between entiities Customer orders Order Order has item Supplier supplies item Everything has to be connected to at least one other table! o Simple ERD Customer Has Order o Attributes: characteristics that describe entities Customer: Number, Name, Phone Supplier: Number, Name, Phone o Cardinality: the maximum number of occurences of one entity for a single occurrence of the related entity E.g. Customers to orders, customers to invoices (THESE ARE BUSINESS RULES!) How to tell: think "each instance of ENTITY1
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Unformatted text preview: can RELATIONSHIP at most how many instances of ENTITY2's Or: each CUSTOMER can ORDER Exercises o Order - has - item You can have multiple items on one order A bag of flour can be on more than one order (a vehicle by vin # could not be on more than one order however o Item - supplied by - supplier What's the problem with M:M relationships? o Really confusing! o Create a linking table Always use singular when naming a table and working with these. Mapping o Sequence Map entities Map relationships 1:M 1:1 M:M o For 1:M The primary key in the 1 side becomes the foreign key on the many side o For 1:1 Check to see if there's an optional side The primary key on the required side becomes the foreign key in the optional side...
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This document was uploaded on 04/04/2011.

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ISYS DAY 4 - can RELATIONSHIP at most how many instances of...

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