Chapter 7 Relational Database Design by ER and EERR toRelational Mapping Chapter Outline ER to Relational Mapping Algorithm Step 1 Mapping of Regular Entity Types Step 2 Mapping of Weak Entity Types Step 3 Mapping of Binary 1 1 Relation Types Step 4 Mapping of Binary 1 N Relationship Types Step 5 Mapping of Binary M N Relationship Types Step 6 Mapping of Multivalued attributes Step 7 Mapping of N ary Relationship Types Mapping EER Model Constructs to Relations Step 8 Options for Mapping Specialization or Generalization Step 9 Mapping of Union Types Categories ER to Relational Mapping Algorithm Step 1 Mapping of Regular Entity Types For each regular strong entity type E in the ER schema create a relation R that includes all the simple attributes of E Choose one of the key attributes of E as the primary key for R If the chosen key of E is composite the set of simple attributes that form it will together form the primary key of R Example We create the relations EMPLOYEE DEPARTMENT and PROJECT in the relational schema corresponding to the regular entities in the ER diagram SSN DNUMBER and PNUMBER are the primary keys for the relations EMPLOYEE DEPARTMENT and PROJECT as shown FIGURE 7 1 The ER conceptual schema diagram for the COMPANY database FIGURE 7 2 Result of mapping the COMPANY ER schema into a relational schema ER to Relational Mapping Algorithm contd Step 2 Mapping of Weak Entity Types For each weak entity type W in the ER schema with owner entity type E create a relation R include all simple attributes or simple components of composite attributes of W as attributes of R Also include as foreign key attributes of R the primary key attribute s of the relation s that correspond to the owner entity type s The primary key of R is the combination of the primary key s of the owner s and the partial key of the weak entity type W if any Example Create the relation DEPENDENT in this step to correspond to the weak entity type DEPENDENT Include the primary key SSN of the EMPLOYEE relation as a foreign key attribute of DEPENDENT renamed to ESSN The primary key of the DEPENDENT relation is the combination ESSN DEPENDENT NAME because DEPENDENT NAME is the partial key of DEPENDENT ER to Relational Mapping Algorithm contd Step 3 Mapping of Binary 1 1 Relation Types For each binary 1 1 relationship type R in the ER schema identify the relations S and T that correspond to the entity types participating in R There are three possible approaches 1 Foreign Key approach Choose one of the relations say S and include a foreign key in S the primary key of T It is better to choose an entity type with total participation in R in the role of S 1 2 Example 1 1 relation MANAGES is mapped by choosing the participating entity type DEPARTMENT to serve in the role of S because its participation in the MANAGES relationship type is total Merged relation option An alternate mapping of a 1 1 relationship type is possible by merging the two entity types and the relationship into a single relation This may be appropriate when both participations are total Cross reference or relationship relation option The third alternative is to set up a third relation R for the purpose of cross referencing the primary keys of the two relations S and T representing the entity types ER to Relational Mapping Algorithm contd Step 4 Mapping of Binary 1 N Relationship Types For each regular binary 1 N relationship type R identify the relation S that represent the participating entity type at the N side of the relationship type Include as foreign key in S the primary key of the relation T that represents the other entity type participating in R Include any simple attributes of the 1 N relation type as attributes of S Example 1 N relationship types WORKS FOR CONTROLS and SUPERVISION in the figure For WORKS FOR we include the primary key DNUMBER of the DEPARTMENT relation as foreign key in the EMPLOYEE relation and call it DNO ER to Relational Mapping Algorithm contd Step 5 Mapping of Binary M N Relationship Types For each regular binary M N relationship type R create a new relation S to represent R Include as foreign key attributes in S the primary keys of the relations that represent the participating entity types their combination will form the primary key of S Also include any simple attributes of the M N relationship type or simple components of composite attributes as attributes of S Example The M N relationship type WORKS ON from the ER diagram is mapped by creating a relation WORKS ON in the relational database schema The primary keys of the PROJECT and EMPLOYEE relations are included as foreign keys in WORKS ON and renamed PNO and ESSN respectively Attribute HOURS in WORKS ON represents the HOURS attribute of the relation type The primary key of the WORKS ON relation is the combination of the foreign key attributes ESSN PNO ER to Relational Mapping Algorithm contd Step 6 Mapping of Multivalued attributes For each multivalued attribute A create a new relation R This relation R will include an attribute corresponding to A plus the primary key attribute K as a foreign key in R of the relation that represents the entity type of relationship type that has A as an attribute The primary key of R is the combination of A and K If the multivalued attribute is composite we include its simple components Example The relation DEPT LOCATIONS is created The attribute DLOCATION represents the multivalued attribute LOCATIONS of DEPARTMENT while DNUMBER as foreign keyrepresents the primary key of the DEPARTMENT relation The primary key of R is the combination of DNUMBER DLOCATION ER to Relational Mapping Algorithm contd Step 7 Mapping of N ary Relationship Types For each n ary relationship type R where n 2 create a new relationship S to represent R Include as foreign key attributes in S the primary keys of the relations that represent the participating entity types Also include any simple attributes of the n ary relationship type or simple components of composite attributes as attributes of S Example The relationship type SUPPY in the ER on the next slide This can be mapped to the relation SUPPLY shown in the relational schema whose primary key is the combination of the three foreign keys SNAME PARTNO PROJNAME FIGURE 4 11 Ternary relationship types a The SUPPLY relationship FIGURE 7 3 Mapping the n ary relationship type SUPPLY from Figure 4 11a Summary of Mapping constructs and constraints Table 7 1 Correspondence between ER and Relational Models ER
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