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18 Cards in this Set
- Front
- Back
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What is data independence?
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This allows applications to be insulated from changes in the way the data is structured and stored
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What are the main benefits of using a dbms to manage data in applications involving extensive data access? 1.1 and 1.4
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Data independence, efficient data access (dbms can retrieve faster than other prog methods), Data integrity and security (data protected by DBMS), Data admin, concurrent access & crash recovery (users protected by DBMS sched & rollback), reduced application development time
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When would you store data in a DBMS instead of in operating system files and vice-versa? 1.3
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If data requires lots of space and quick retrieval a operating system file retrieval would be to slow. Also, multiple users should not be aware of each other and a DBMS can manage this.
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Explain the advantages of using a query language instead of custom programs to process data? 1.6
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Each query scenario would have to be programmed for separately.
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What is a transaction? What guarantees does a DBMS offer with respect to transactions?1.7
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A transaction is any one execution of a user program in a DBMS. A DBMS will write the transaction before executing it so, if interupted, it can roll it back.
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What are locks ina DBMS, and why are they used? What is checkpointing and why is it used? 1.7
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A record lock prevents other users from altering the same record at the same time. A DBMS schedules the locs.
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Identify the main components in a DBMS and briefly explain what they do. 1.8
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The parser feeds the optimizer. A query optimizer uses info about how the data is stored to produce an efficient execution plan that the plan executer uses. Recovery manager ensures Atomicity and Durability. The buffer manager brings page in from disk to main memory as needed in response to read requests. Concurrency control is provided by Transaction Manager and Lock Manager.
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Name the main steps in database design. What is the goal of each step? In which step is the ER model mainly used? 2.1
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1) Requirements Analysis 2) Conceptual Database Design converts requirements into ER diagram 3) Logical database designs converts the ER diagram to relational database schema.
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Define these terms: entity, entity set, descriptive attributes. 2.3
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An entity is an object in the real world that is distinguishable from other objects. An entity set is a collection of similar entities. An entity is described using a set of attributes
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Define these terms: relationship, relationship set, descriptive attributes. 2.3
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A relationship is an association among two or more entities. A collection is a relationship set. A descriptive attribute is used to record information about the relationship, rather than one of the entities.
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What guidelines would you use for deciding between an attribute or entity set? 2.5
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The complexity of the data and how it will be used. Address can be an attribute if it will not be used. If it is going to be used and you need multiple addresses and the ability to query by zip, state, city, etc, consider an entity set.
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What guidelines would you use for deciding between an entity or a relationship set? 2.5
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Using a relationship can result in redundant data storage when the relationship attributes are varying with the One entity in a One to Many relationship.
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What guidelines would you use for deciding between a ternary or binary relationship? 2.5
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Ternary is not always better with multiple entities that have different participation requirements.
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What guidelines would you use for deciding whether to use an aggregation? 2.5
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An aggregation allows you to summarize the repeated data and use an entity set like an entity. The choice between using an aggregation or a ternary relationship is mainly determined by the existence of a relationship that relates a relationship set to an entity set or second relationship set.
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What is the difference between specialization and generalization in class hierarchies?
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A generalization is traveling up a ISA hierarchy from contract employee to employee; A specialization is traveling down the hierarchy from employee to contract employee.
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What is the difference between a relation schema and a relation instance?
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A schema is the overall design of a relation. An instance is the set of data in the relation at any point in time.
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What is a view and how does it support logical data independence?
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A view is a table whose rows are not explicitly stored in the database but are computed as needed from a view definition. It supports logical data independence because because it can be used to define relations in the external schema that mask changes in the conceptual schema.
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Why does SQL restrict the class of views that can be updated?
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So that partial records that can't be uniquely identified aren't added to underlying tables???
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