Advanced Databases (SE487) Prince Sultan University College of Computer and Information Sciences

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1 Advanced Databases (SE487) Prince Sultan University College of Computer and Information Sciences ER to Relational Mapping Anis Koubaa Chapter 9 Outline } Relational Database Design Using ER-to-Relational Mapping 2 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20

2 Acknowledgement } Slides are combined from several sources including: } Textbook slides of Fundamentals of Database Systems, 6th Edition, Addison-Wesley. by Elmasri/avathe (Chapter 7) } Course by Instructor Prof. Feifei Li (University of Utah) } Textbook slides of Modern Database Management, th Edition, Jeffrey A. Hoffer, V. Ramesh, Heikki Topi 3 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 Relational Database Design by ER- and EER-to-Relational Mapping } Design a relational database schema } Based on a conceptual schema design } Seven-step algorithm to convert the basic ER model constructs into relations } Additional steps for EER model 4 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 2

3 Relational Database Design Using ERto-Relational Mapping Figure 9. The ER conceptual schema diagram for the COMPAY database. Fname Minit Lname Bdate ame Address Salary Ssn Sex WORKS_FOR ame Locations umber Start_date umber_of_employees DEPARTMET MAAGES COTROLS Supervisor SUPERVISIO M WORKS_O Supervisee Hours ame DEPEDETS_OF umber Location DEPEDET ame Sex Birth_date Relationship 5 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 Fname Minit Lname Ssn Bdate Address Sex Salary Super_ssn Dno DEPARTMET Dname Dnumber Mgr_ssn Mgr_start_date DEPT_LOCATIOS Dnumber Dlocation Pname Pnumber Plocation Dnum WORKS_O Essn Pno Hours DEPEDET Essn Dependent_name Sex Bdate Relationship Figure 9.2 Result of mapping the COMPAY ER schema into a relational database schema. 6 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 3

4 Step : Mapping of Regular Entity Types. Step : Mapping of Regular Entity Types } COMPAY database example } Assume that the mapping will create tables with simple singlevalued attributes 8 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 4

5 Step : Mapping of Regular Entity Types } Step : Mapping of Regular Entity Types } For each regular entity type, create a relation R that includes all the simple attributes of E } Called entity relations } Each tuple represents an entity instance 9 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 Step : Mapping of Regular Entity Types Figure 9.3 Illustration of some mapping steps. (a) Entity relations after step. (b) Additional weak entity relation after step 2. (c) Relationship relation after step 5. (d) Relation representing multivalued attribute (a) Fname Minit Lname Ssn Bdate Address Sex Salary DEPARTMET Dname Dnumber Pname Pnumber Plocation Figure 9. The ER conceptual schema diagram for the COMPAY database. Fname Minit Lname Bdate ame Address Salary Ssn Sex WORKS_FOR ame Locations umber Start_date umber_of_employees DEPARTMET MAAGES Hours M WORKS_O Supervisor Supervisee SUPERVISIO ame DEPEDETS_OF umber COTROLS Location DEPEDET 0 ame Sex Birth_date Relationship Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 5

6 Step 2: Mapping of Weak Entity Types Step 2: Mapping of Weak Entity Types } Step 2: Mapping of Weak Entity Types } For each weak entity type, create a relation R and include all simple attributes of the entity type as attributes of R } Include primary key attribute of owner as foreign key attributes of R 2 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 6

7 Step 2: Mapping of Weak Entity Types Figure 9.3 Illustration of some mapping steps. (a) Entity relations after step. (b) Additional weak entity relation after step 2. (c) Relationship relation after step 5. (d) Relation representing multivalued attribute after step 6. (a) (b) Fname Minit Lname Ssn Bdate Address Sex Salary DEPARTMET Dname Dnumber Pname Pnumber Plocation DEPEDET Essn Dependent_name Sex Bdate Relationship ame umber Start_date umber_of_employees DEPARTMET MAAGES Hours M WORKS_O Supervisor Supervisee SUPERVISIO ame DEPEDETS_OF umber COTROLS Location DEPEDET 3 ame Sex Birth_date Relationship Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 Step 3: Mapping of Binary : 7

8 Step 3: Mapping of Binary : } Step 3: Mapping of Binary : } For each binary : relationship type } Identify relations that correspond to entity types participating in R } Possible approaches: } Foreign key approach (most useful) } Merged relationship approach } Cross-reference or relationship relation approach 5 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 Step 3: Mapping of Binary : } Step 3: Mapping of Binary : } Foreign key approach: Choose one of the relations S, say and include as a foreign key in S the primary key of T. } } } We map the : relationship type MAAGES from Figure 9. by choosing the participating entity type DEPARTMET to serve in the role of S because its participation in the MAAGES relationship type is total (every department has a manager). We include the primary key of the relation as foreign key in the DEPARTMET relation and rename it Mgr_ssn. We also include the simple attribute Start_date of the MAAGES relationship type in the DEPARTMET relation and rename it Mgr_start_date Figure 9. The ER conceptual schema diagram for the COMPAY database. Fname Minit Lname Bdate Ssn T Supervisor ame Address SUPERVISIO ame DEPARTMET Salary Sex Supervisee Start_date MAAGES M DEPEDETS_OF DEPEDET WORKS_FOR umber_of_employees WORKS_O Hours ame umber Sex Birth_date Relationship ame Locations DEPARTMET COTROLS Location umber Dname Dnumber Mgr_ssn Mgr_start_date S 6 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 8

9 Step 3: Mapping of Binary : } Step 3: Mapping of Binary : } Merged relation approach: An alternative mapping of a : relationship type is to merge the two entity types and the relationship into a single relation. } This is possible when both participations are total, as this would indicate that the two tables will have the exact same number of tuples at all times. 7 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 Step 3: Mapping of Binary : } Step 3: Mapping of Binary : } Cross-reference or relationship relation approach: 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 } This approach is required for binary M: relationships. } The relation R is called a relationship relation (or sometimes a lookup table) 8 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 9

10 Step 4: Mapping of Binary : Step 4: Mapping of Binary : } Step 4: Mapping of Binary : } For each regular binary : relationship type } Identify relation that represents participating entity type at -side of relationship type } Include primary key of other entity type as foreign key in S } Include simple attributes of : relationship type as attributes of S } Alternative approach } Use the relationship relation (crossreference) option as in the third option for binary : relationships Figure 9. The ER conceptual schema diagram for the COMPAY database. Fname Minit Lname Bdate Ssn Supervisor ame Address SUPERVISIO ame Salary Sex Supervisee Start_date MAAGES M DEPEDETS_OF DEPEDET WORKS_FOR umber_of_employees WORKS_O Hours ame umber Sex Birth_date Relationship ame Locations DEPARTMET COTROLS Location umber Fname Minit Lname Ssn Bdate Address Sex Salary Super_ssn Dno Pname Pnumber Plocation Dnum 20 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 0

11 Step 5: Mapping of Binary M: Step 5: Mapping of Binary M: } Step 5: Mapping of Binary M: } For each binary M: relationship type } Create a new relation S } Include primary key of participating entity types as foreign key attributes in S } Include any simple attributes of M: relationship type Figure 9.3 Illustration of some mapping steps. (a) Entity relations after step. (b) Additional weak entity relation after step 2. (c) Relationship relation after step 5. (d) Relation representing multivalued attribute after step 6. (a) (b) Fname Minit Lname Ssn Bdate Address Sex Salary DEPARTMET Dname Dnumber Pname Pnumber Plocation DEPEDET Essn Dependent_name Sex Bdate Relationship (c) WORKS_O Essn Pno Hours 22 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20

12 Step 6: Mapping of Multivalued Attributes Step 6: Mapping of Multivalued Attributes } Step 6: Mapping of Multivalued Attributes } For each multivalued attribute A } Create a new relation R } Primary key of R is the combination of A and K (key of relation of A) } If the multivalued attribute is composite, include its simple components } we create a relation DEPT_LOCATIOS (see Figure 9.3(d)). The attribute Dlocation represents the multivalued attribute LOCATIOS of DEPARTMET, while Dnumber as foreign key represents the primary key of the DEPARTMET relation. The primary key of DEPT_LOCATIOS is the combination of {Dnumber, Dlocation}. A separate tuple will exist in DEPT_LOCATIOS for each loca- tion that a department has. (d) DEPT_LOCATIOS Dnumber Dlocation 24 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 2

13 Step 7: Mapping of -ary Step 7: Mapping of -ary Relationship Types } Step 7: Mapping of -ary } For each n-ary relationship type R } Create a new relation S to represent R } Include primary keys of participating entity types as foreign keys } Include any simple attributes as attributes Figure 9.4 Mapping the n-ary relationship type SUPPLY from Figure 7.7(a). SUPPLIER Sname... Proj_name... Sname SUPPLIER Quantity SUPPLY Part_no PART Proj_name PART Part_no... SUPPLY Sname Proj_name Part_no Quantity 26 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 3

14 Discussion and Summary of Mapping for ER Model Constructs Table 9. Correspondence between ER and Relational Models ER MODEL RELATIOAL MODEL Entity type Entity relation : or : relationship type Foreign key (or relationship relation) M: relationship type Relationship relation and two foreign keys n-ary relationship type Relationship relation and n foreign keys Simple attribute Attribute Composite attribute Set of simple component attributes Multivalued attribute Relation and foreign key Value set Domain Key attribute Primary (or secondary) key 27 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 Summary } Map conceptual schema design in the ER model to a relational database schema } Algorithm for ER-to-relational mapping } Illustrated by examples from the COMPAY database } Include additional steps in the algorithm for mapping constructs from EER model into relational model 28 Elmasri & avathe, (c) Pearson Education - Addison-Wesley, 20 4