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Database Management Chapter 1 Essay

859 words - 4 pages

Principles of Database Management Systems

Homework 1

Ying Li

4861521

Homework 1

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?

Answer:
External schemas, which usually are also in terms of the data model of the DBMS, allow data access to be customized (and authorized) at the level of individual users or groups of users.
Internal (physical) schemas summarize how the relations described in the conceptual schema are actually stored on secondary storage devices such as disks and tapes.
Conceptual (logical) schema describes the ...view middle of the document...

Answer:
A security facility is necessary because Scrooge does not plan to share his list with anyone else. Although he is running it on his stand-alone PC, a rival could break in and attempt to query his database. The security features of database would prevent the intruder.

Concurrency control is not needed because only he himself uses the database.

Crash recovery is essential for any database; Scrooge would not want to lose his data if the power was interrupted while he was using the system.

A view mechanism is needed. Scrooge could use this to develop “custom screens” that he could conveniently bring up without writing long queries repeatedly.

A query language is necessary since Scrooge must be able to analyze the dark secrets of his victims. In particular, the query language is also used to define views.

Exercise 1.9
Answer the following questions:
1. What is a transaction?
2. Why does a DBMS interleave the actions of different transactions instead of executing transactions one after the other?
3. What must a user guarantee with respect to a transaction and database consistency? What should a DBMS guarantee with respect to concurrent execution of several transactions and database consistency?
4. Explain the strict two-phase locking protocol.
5. What is the WAL property, and why is it important?

Answer:
1. A transaction is anyone execution of a user program in a user program in a DBMS. This is the basic unit of change as seen by the DBMS.

2. A DBMS is typically shared among many users. Transactions from these users can be interleaved to improve the execution time of users’ queries. By interleaving...

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