tailieunhanh - Database Design Using Entity-Relationship Diagrams phần 3

Chúng ta có thể thấy MALL đó có một thuộc tính đa giá trị, store_names. Điều này không làm cho bảng trên một bảng quan hệ bởi vì store_names không phải là nguyên tử - đó là nhiều giá trị. Đối với các thuộc tính đa giá trị, các quy tắc lập bản đồ là: M1c. Đối với các thuộc tính đa giá trị, tạo thành một bảng riêng biệt cho các thuộc tính đa giá trị. | We can see that MALL has a multi-valued attribute store_names. This does not make the above table a relational table because store_names is not atomic it is multi-valued. For multi-valued attributes the mapping rule is M1c. For multi-valued attributes form a separate table for the multi-valued attribute. Record a row for each value of the multivalued attribute together with the key from the original table. Remove the multi-valued attribute from the original table. Using this mapping rule the above data would be mapped to two relations a relation with the multi-valued attribute and a relation with the multi-valued attribute excised. Relation with the Multi-Valued Attribute MALL-Store name store_name West Florida Mall West Florida Mall West Florida Mall West Florida Mall Cordova Mall Cordova Mall Cordova Mall Cordova Mall Navy Mall Navy Mall Navy Mall Penney s Sears Dollar Store Rex Dillards Parisian Circuit City Radio Shack Belks Wards Pearle Vision MALL-Store name store_name Navy Mall McRaes Navy Mall Sears BelAir Mall Dillards BelAir Mall Sears BelAir Mall Penney s BelAir Mall Best Buy Bel Air Mall Pizza Hut Relation with the Multi-Valued Attribute Excised MALL name address West Florida Mall N Davis Hwy Pensacola FL Cordova Mall 9th Avenue Pensacola FL Navy Mall BelAir Mall Navy Blvd Pensacola FL 10th Avenue Mobile AL Our relational database maps to Note The primary keys are underlined. Chapter 3 Beyond the First Entity Diagram Overview Now that we have devised a method for drawing interpreting and refining one primary entity we need to move to more complex databases. To progress from here we continue with our primary entity and look for other information that would be associated with related to that entity. The first technique employed in this chapter is methodical we test our primary entity to see whether or not our attributes should be entities themselves. We will then look for other pieces of information in our description add them to 1 an existing entity and