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a relationship in which a foreign key refers to the same table Selfreferencing relation ships represent associa tions among members of the same set
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FacSSN 098-76-5432 543-21-0987 654-32-1098 765-43-2109 876-54-3210 987-65-4321
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Sample Faculty Table
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FacFirstName LEONARD VICTORIA LEONARD NICKI CRISTOPHER JULIA FacLastName VINCE EMMANUEL FIBON MACON COLAN MILLS FacCity SEATTLE BOTHELL SEATTLE BELLEVUE SEATTLE SEATTLE FacState WA WA WA WA WA WA FacDept MS MS MS FIN MS FIN FacRank ASST PROF ASSC PROF ASST ASSC FacSalary $35,000 $120,000 $70,000 $65,000 $40,000 $75,000 FacSupervisor 654-32-1098 543-21-0987 654-32-1098 765-43-2109 FacHireDate 01-Apr-95 01-Apr-96 01-Apr-95 01-Apr-97 01 -Apr-99 01-Apr-00 FacZipCode 98111-9921 98011-2242 98121-0094 98015-9945 98114-1332 98114-9954
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The Relational Data Model
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(second row) supervises Leonard Fibon (third row) The FacSupervisor column shows this relationship: the FacSupervisor value in the third row (543-21-0987) matches the FacSSN value in the second row A referential integrity constraint involving the FacSupervisor column represents the self-referencing relationship In the CREATE TABLE statement, the referential integrity constraint for a self-referencing relationship can be written the same way as other referential integrity constraints CREATE TABLE Faculty FacSSN CHAR(11), ( FacFirstName VARCHAR(50) CONSTRAINT FacFirstNameRequired NOT NULL, VARCHAR(50) CONSTRAINT FacLastNameRequired NOT NULL, FacLastName FacCity CONSTRAINT FacCityRequired NOT NULL, VARCHAR(50) CONSTRAINT FacStateRequired NOT NULL, FacState CHAR(2) CONSTRAINT FacZipCodeRequired NOT NULL, CHAR(10) FacZipCode DATE, FacHireDate FacDept CHAR(6), FacRank CHAR(4), FacSalary DECIMALS 0,2), FacSupervisor CHAR(11), CONSTRAINT PKFaculty PRIMARY KEY (FacSSN), CONSTRAINT FKFacSupervisor FOREIGN KEY (FacSupervisor) REFERENCES Faculty
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In recent years, commercial DBMSs have provided graphical representations for referential integrity constraints The graphical representation makes referential integrity easier to define and understand than the text representation in the CREATE TABLE statement In addition, a graphical representation supports nonprocedural data access To depict a graphical representation, let us study the Relationship window in Microsoft Access Access provides the Relationship window to visually define and display referential integrity constraints Figure 32 shows the Relationship window for the tables of the university database Each line represents a referential integrity constraint or relationship In a relationship, the primary key table is known as the parent or " 1 " table (for example,
FIGURE 32 Relationship Window for the University Database
Relationships
FacSSN FacFirstName FacLastName FacCity FacState FacDept FacRank FacSalary FacSupervisor FacHireDate FacZipCode
Connections are from the primary key (bold font) to the foreign key
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Part Two
Understanding Relational Databases
1-M relationship
a connection between two tables in which one row of a parent table can be referenced by many rows of a child table 1-M relationships are the most common kind of relationship
M-N relationship
a connection between two tables in which rows of each table can be related to many rows of the other table M-N relationships cannot be directly represented in the Relational Model Two 1-M relationships and a linking or associative table represent an M-N relationship
Student) and the foreign key table (for example, Enrollment) is known as the child or " M " (many) table The relationship from Student to Enrollment is called "1-M" (one to many) because a student can be related to many enrollments but an enrollment can be related to only one student Similarly, the relationship from the Offering table to the Enrollment table means that an offering can be related to many enrollments but an enrollment can be related to only one offering You should practice by writing similar sentences for the other relationships in Figure 32 M-N (many to many) relationships are not directly represented in the Relational Model An M-N relationship means that rows from each table can be related to many rows of the other table For example, a student enrolls in many course offerings and a course offering contains many students In the Relational Model, a pair of 1-M relationships and a linking or associative table represents an M-N relationship In Figure 32, the linking table Enrollment and its relationships with Offering and Student represent an M-N rela tionship between the Student and Offering tables Self-referencing relationships are represented indirectly in the Relationship window The self-referencing relationship involving Faculty is represented as a relationship between the Faculty and Faculty_1 tables Faculty_1 is not a real table as it is created only inside the Access Relationship window Access can only indirectly show self-referencing relationships A graphical representation such as the Relationship window makes it easy to identify tables that should be combined to answer a retrieval request For example, assume that you want to find instructors who teach courses with "database" in the course description Clearly, you need the Course table to find "database" courses You also need the Faculty table to display instructor data Figure 32 shows that you also need the Offering table because Course and Faculty are not directly connected Rather, Course and Faculty are connected through Offering Thus, visualizing relationships helps to identify tables needed to fulfill retrieval requests Before attempting the retrieval problems in later chapters, you should carefully study a graphical representation of the relationships You should construct your own diagram if one is not available
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