Problem

Exercise 1.4 Explain the difference between external, internal, and conceptual schemas. How are these different schema layers related to the concepts of logical and physical data independence?

Step-by-step solution

1. Step 1 of 2

   • External Schema: represents data accessed by end users or application programs, it provides customized information to the end users.

   • Conceptual Schema: represent the logical structure of the data, how data is stored in a database is represented by conceptual schema. Conceptual schema provides information to external schema which is customized and is accessed by application programs.

   • Internal Schema: represents the physical storage of data on a disk or a physical storage device. Conceptual schema maps internal schema data to the external schema.

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2. Step 2 of 2

   • Physical data independence is achieved by internal or physical schema, by insulating information like how data is physically stored on a device. Physical data independence insulates the physical storage changes from conceptual schema.

   • Logical data independence is achieved by conceptual schema, by insulating information related to the logical structure of the data. Logical data independence insulates logical structure changes from the user or external schema.

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****** Describe the problems with the traditional two-tier client-server architecture and discuss how these problems were overcome with the three-tier client-server architecture.

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  All projects are mostly divided in to two categories like two tier and three tier architectures. Two tier architecture is client server based application and three tier architecture is web based application. The main problem with the two tier architecture is if user sends the requests multiple times server will not be in the position of responding the requests at the same time so that it will lead to the data integrity issue. The performance in this will degrade upon the increasing number of users. In three tier archtecture these are overcome because the insertrion, update, delete commands are written before itself in the validation time by using layers which are present in it so there will not be problem with data integrity. Presentation layers will look after the requests that come from the users even though it is multiple.

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