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Unformatted text preview: neral rule of thumb: A table should not pertain to more than one entity type 40 Example–Figure 5­2b Question–Is this a relation? Answer–Yes: Unique rows and no multivalued attributes Question–What’s the primary key? Answer–Composite: Emp_ID, Course_Title 41 Anomalies in this Table Insertion–can’t enter a new employee without having the employee take a class Deletion–if we remove employee 140, we lose information about the existence of a Tax Acc class Modification–giving a salary increase to employee 100 forces us to update multiple records Why do these anomalies exist? Because there are two themes (entity types) in this one relation. This results in data duplication and an unnecessary dependency between the entities 42 Functional Dependencies and Keys Functional Dependency: The value of one attribute (the determinant) determines the value of another attribute Candidate Key: A unique identifier. One of the candidate keys will become the primary key E.g. perhaps there is both credit card number and SS# in a table…in this case both are candidate keys Each non­key field is functionally dependent on every candidate key 43 Figure 5.22 Steps in normalization 44 First Normal Form No multivalued attributes Every attribute value is atomic Fig. 5­25 is not in 1st Normal Form (multivalued attributes) it is not a relation Fig. 5­26 is in 1st Normal form All relations are in 1st Normal Form 45 Table with multivalued attributes, not in 1st normal form Note: this is NOT a relation 46 Table with no multivalued attributes and unique rows, in 1st normal form Note: this is relation, but not a well-structured one 47 Anomalies in this Table Insertion–if new product is ordered for order 1007 of existing customer, customer data must be re­ entered, causing duplication Deletion–if we delete the Dining Table from Order 1006, we lose information concerning this item's finish and price Update–changing the price of product ID 4 requires update in several records Why do these anomalies exist? Because there are multiple themes (entity types) in one relation. This results in duplication and an unnecessary dependency between the entities 48 Second Normal Form 1NF PLUS every non­key attribute is fully functionally dependent on the ENTIRE primary key Every non­key attribute must be defined by the entire key, not by only part of the key No partial functional dependencies 49 Figure 5-27 Functional dependency diagram for INVOICE Order_ID Order_Date, Customer_ID, Customer_Name, Customer_Address Customer_ID Customer_Name, Customer_Address Product_ID Product_Description, Product_Finish, Unit_Price Order_ID, Product_ID Order_Quantity Therefore, NOT in 2nd Normal Form 50 Figure 5-28 Removing partial dependencies Getting it into Second Normal Form Partial dependencies are removed, but there are still transitive dependencies 51 Third Normal Form 2NF PLUS no transitive dependencies (functional dependencies on non­primary­key attributes) Note: This is called transitive, because the primary key is a determinant for another attribute, which in turn is a determinant for a third Solution: Non­key determinant with transitive dependencies go into a new table; non­key determinant becomes primary key in the new table and stays as foreign key in the old table 52 Figure 5-28 Removing partial dependencies Getting it into Third Normal Form Transitive dependencies are removed 53 Merging Relations View Integration–Combining entities from multiple ER models into common relations Issues to watch out for when merging entities from different ER models: Synonyms–two or more attributes with different names but same meaning Homonyms–attributes with same name but different meanings Transitive dependencies–even if relations are in 3NF prior to merging, they may not be after merging Supertype/subtype relationships–may be hidden prior to merging 54 Enterprise Keys Primary keys that are unique in the whole database, not just within a single relation Corresponds with the concept of an object ID in object­oriented systems 55 Figure 5-31 Enterprise keys a) Relations with enterprise key b) Sample data with enterprise key 56...
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