Attribute Specification in Select Clause




The asterisk symbol “ * ” can be used in the select clause to denote “all attributes.”
Thus, the use of instructor.* in the select clause of the query:

select instructor.*
from instructor, teaches
where instructor.ID= teaches.ID;

indicates that all attributes of instructor are to be selected. A select clause of the
form select * indicates that all attributes of the result relation of the from clause
are selected.



Frequently Asked Questions

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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: An entity type DEPARTMENT with attributes Name, Number, Locations, Manager, and Manager_start_date. Locations is the only multivalued attribute. We can specify that both Name and Number are (separate) key attributes because each was specified to be unique view more..
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Ans: A database usually contains groups of entities that are similar. For example, a company employing hundreds of employees may want to store similar information concerning each of the employees. These employee entities share the same attributes, but each entity has its own value(s) for each attribut view more..
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Ans: Select clause uses. view more..
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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: 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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