Refining the ER Design for the COMPANY Database




We can now refine the database design in Figure 7.8 by changing the attributes that represent relationships into relationship types. The cardinality ratio and participation constraint of each relationship type are determined from the requirements listed in Section 7.2. If some cardinality ratio or dependency cannot be determined from the requirements, the users must be questioned further to determine these structural constraints. In our example, we specify the following relationship types:

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? MANAGES, a 1:1 relationship type between EMPLOYEE and DEPARTMENT. EMPLOYEE participation is partial. DEPARTMENT participation is not clear from the requirements. We question the users, who say that a department must have a manager at all times, which implies total participation.13 The attribute Start_date is assigned to this relationship type.

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? WORKS_FOR, a 1:N relationship type between DEPARTMENT and EMPLOYEE. Both participations are total.

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? CONTROLS, a 1:N relationship type between DEPARTMENT and PROJECT. The participation of PROJECT is total, whereas that of DEPARTMENT is determined to be partial, after consultation with the users indicates that some departments may control no projects.

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? SUPERVISION, a 1:N relationship type between EMPLOYEE (in the supervisor role) and EMPLOYEE (in the supervisee role). Both participations are determined to be partial, after the users indicate that not every employee is a supervisor and not every employee has a supervisor.

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? WORKS_ON, determined to be an M:N relationship type with attribute Hours, after the users indicate that a project can have several employees working on it. Both participations are determined to be total.

? DEPENDENTS_OF, a 1:N relationship type between EMPLOYEE and DEPENDENT, which is also the identifying relationship for the weak entitytype DEPENDENT. The participation of EMPLOYEE is partial, whereas that of DEPENDENT is total.

After specifying the above six relationship types, we remove from the entity types in Figure 7.8 all attributes that have been refined into relationships. These include Manager and Manager_start_date from DEPARTMENT; Controlling_department from PROJECT; Department, Supervisor, and Works_on from EMPLOYEE; and Employee from DEPENDENT. It is important to have the least possible redundancy when we design the conceptual schema of a database. If some redundancy is desired at the storage level or at the user view level, it can be introduced later.



Frequently Asked Questions

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Ans: Entity types that do not have key attributes of their own are called weak entity types. In contrast,regular entity types that do have a key attribute—which include all the examples discussed so far—are called strong entity types view more..
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Ans: Select clause uses. view more..
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Ans: There are several implicit relationships among the various entity types. In fact, whenever an attribute of one entity type refers to another entity type, some relationship exists. For example, the attribute Manager of DEPARTMENT refers to an employee who manages the department; the attribute view more..
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Ans: If some cardinality ratio or dependency cannot be determined from the requirements, the users must be questioned further to determine these structural constraints view more..
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Ans: The individual entity instances in an entity set and the individual relationship instances in a relationship set. In ER diagrams the emphasis is on representing the schemas rather than the instances. view more..
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Ans: The UML methodology is being used extensively in software design and has many types of diagrams for various software design purposes. We only briefly present the basics of UML class diagrams here, and compare them with ER diagrams view more..
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Ans: we defined the degree of a relationship type as the number of participating entity types and called a relationship type of degree two binary and a relationship type of degree three ternary view more..
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Ans: SQL offers the user some control over the order in which tuples in a relation are displayed. view more..
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Ans: The rapid advancement of the use of information technology (IT) in industry, government, and academia raises challenging questions and problems regarding the protection and use of personal information. Questions of who has what rights to information about individuals for which purposes become more important as we move toward a world in which it is technically possible to know just about anything about anyone. view more..
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Ans: Sensitivity of data is a measure of the importance assigned to the data by its owner, for the purpose of denoting its need for protection. Some databases contain only sensitive data while other databases may contain no sensitive data at all. view more..
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Ans: Whenever a person or a group of persons needs to access a database system, the individual or group must first apply for a user account. The DBA will then create a new account number and password for the user if there is a legitimate need to access the database. view more..
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Ans: The database administrator (DBA) is the central authority for managing a database system. The DBA’s responsibilities include granting privileges to users who need to use the system and classifying users and data in accordance with the policy of the organization view more..
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Ans: SQL includes a between comparison operator to simplify where clauses that specify that a value be less than or equal to some value and greater than or equal to some other value. view more..
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Ans: Four main control measures are used to provide security of data in databases: Access control, Inference control, Flow control, Data encryption view more..
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Ans: The SQL operations union, intersect, and except operate on relations. view more..
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Ans: Null values present special problems in relational operations, including arithmetic operations, comparison operations, and set operations. view more..
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Ans: Aggregate functions view more..
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Ans: The main Idea for the Sampling Algorithm is to select a small sample , one that fits in main memory of the database of transaction view more..




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