E-R Diagram to Relational Schema

Transcription

1 Database Design ICOM 5016 Database Systems Database Design Amir H. Chinaei Department of Electrical and Computer Engineering University of Puerto Rico, Mayagüez Slides are adapted from: The first step to design a database Draw ER diagrams The first step to design a relational schema Derive it from ER diagrams The first step to design a good relational schema Apply normal forms to the existing relational schema (Chapter 7, lecture 13) Database System Concepts, 5th Ed. Silberschatz, Korth and Sudarshan See for conditions on re-use 2 E-R Diagram to Relational Schema General approach is straightforward Each entity set is mapped to a new table (relation) Each attribute is mapped to a new table column Each relationship set is mapped to either new table columns or to a new table Representing Strong Entity Sets Strong entity set E with attributes a 1 to a n is translated to table E with attributes a 1 to a n Therefore, Each entity of entity set E is a tuple in table E Primary key of entity set E is the primary key of table E Example: Recall: A primary key is a minimal set of attribute(s) whose values uniquely determine a particular entity

2 Representing Weak Entity Sets Example: Weak Entity Sets Weak entity set E with attributes a 1 to a n is translated to B# Address table E with attributes a 1 to a n moreover, table E includes has attributes of the identifying relationship set Branch and primary key attributes of entity set for dominating entities Primary key of relation E is the primary key of the weak entity set E which is the combination of primary key attributes of E and primary key attributes of dominating entities Branch B# Address 5 6 Representing Relationship Sets Representing Relationship Sets cont d 1. If the relationship set with attributes a 1 to a n is an identifying relationship set for a weak entity set E, then add a 1 to a n to table E 2. In general, if the general cardinality constraint for a component entity set E is (1,1), then add following columns to table E 3. Otherwise, Attributes of the relationship set Primary key attributes of remaining component entity sets translate relationship set R to table R as follows Columns of table R should include attributes of the relationship set primary key attributes of each component entity set Primary key of table R determined as follows if the general cardinality constraint for a component entity set E is (0,1), then the p.k. of E is the p.k. of R too otherwise, choose primary key attributes of each component entity 7 8 2

3 Example 1: Relationship Sets Example 2: Relationship Sets Price Date provides Married Provides Price Married Husband Wife Date Note that the role names of an entity set should be prefixed to its primary key as the primary key component of the relationship Another Example Aggregation & Multi-Valued Attr. To translate relationship set involving aggregation of R, Contest 11 treat the aggregation like an entity set whose primary key is the primary key of the table for R To translate a multi-valued attribute M of an entity set E, Make a table M with following attributes attribute M, and primary key attributes of E All above attributes form the primary key of table M

4 Example: Aggr./Multi-Valued Attr. Representing Specialization Hobbies Prefers FS prefers (1,1) likes Likes name Hobbies Hobby Contest 12 Object-Oriented Approach Create one table for each entity set, specialized entity sets inherit all attributes of the higher-level entity sets. E/R Approach Create one table for higher-level entity set, and treat specialized entity subsets like weak entity sets. Using Null Values Create only one table for the higher-level entity set, and include attributes of all specialized entity sets too Specialization: OO approach Specialization: E/R approach Advantage. No need to join when retrieving information about salads. Drawback. Needs union operations. Advantage. No need for union operations. Drawback. Needs the join operation when retrieving information about salads

5 Specialization: Using Null Values Representing Generalization Advantage. Save space if the number of foods are significantly more than the number of salads. Drawback. The schema in not in 2 nd NF. Create a table for each lower-level entity set only (OO Approach) Columns of new tables should include Notes Attributes of lower level entity set Attributes of the superset The higher-level entity set can be defined as a view on the tables for the lower-level entity sets The E/R approach or using null values is not practical Example: Generalization Summary of Schema Design (for now) Soup Sandwich Soup Sandwich 1. Create an ER diagram Visualization of the design goals 2. Translate ER to relational schema 3. Determine basic constraints Primary keys, Referential keys (Foreign keys) 4. Determine other constraints FD, MVD, JD, Detect anomalies and decompose 6. Enforce rest of constraints Check/Assertion constraints Triggers