Esquema de banco de dados

Por
Pedro Santos
-
693
4114

QUESTÃO CERTA: Em qualquer modelo de dados, é importante distinguir entre a descrição do banco de dados e o banco de dados de fato. Em relação à modelagem de dados, a descrição do banco de dados é intitulada: esquema.

QUESTÃO CERTA: Em sistemas gerenciados de banco de dados, a independência dos dados refere-se à capacidade de modificar a estrutura lógica ou física do banco, sem a necessidade de uma reprogramação dos programas de aplicativos.

É isso mesmo! A independência dos dados permite que se altere a estrutura do BD (não se preocupe com essa história de lógica ou física por enquanto) sem que sejam necessárias modificações nos programas ou aplicativos.

QUESTÃO ERRADA: Devido à independência de dados, provida pela referida arquitetura, as modificações do “esquema conceitual” — tais como a adição ou a remoção de um tipo de registro (ou item) de dados — não causam ou requerem alterações no “esquema externo” ou nos programas de acesso ao banco de dados.

A arquitetura de três esquemas é utilizada para explicar os conceitos de independência de dados, definida como a capacidade de alterar o esquema de um nível sem ter que alterar o esquema do próximo nível superior.

Há dois tipos de independência:

1. Independência Lógica de Dados: capacidade de alterar o esquema conceitual sem ter que mudar os esquemas externos ou programas da aplicação. O esquema conceitual pode ser alterado para expandir a base de dados, com adição de novos tipos de registros ou reduzir a base de dados removendo um tipo de registro.

2. Independência Física de Dados: capacidade de alterar o esquema interno sem ter que mudar o esquema conceitual ou externo.

A questão peca ao dizer que adição ou remoção de um tipo de registro não causa alteração no esquema externo.

Isso é verdade para tipos de registros remanescentes da alteração. Para tipos novos ou removidos, há sim que se realizar alterações para acomodá-los no esquema externo.

“Adição ou a remoção de um tipo de registro (ou item) de dados” – é no nível interno e não no nível conceitual!!

QUESTÃO CERTA: Considere as seguintes afirmativas sobre sistemas de bancos de dados federados: possuem esquemas globais compartilhados pelas aplicações.

QUESTÃO ERRADA: Independência de dados lógica é a capacidade de modificar o esquema lógico sem que, com isso, qualquer programa de aplicação precise ser reescrito. Modificações no nível lógico são necessárias, ocasionalmente, para aprimorar o desempenho.

Na verdade, é a capacidade de modificar o esquema conceitual (e não o esquema lógico como afirmado).

Independência lógica de dados: é a capacidade de alterar o esquema conceitual sem mudar o esquema externo ou os programas.

Independência física de dados: é a capacidade de alterar o esquema interno sem mudar o esquema conceitual.

Segundo Navathe (2011, p.23) a questão estaria correta da seguinte maneira,

“Independência de dados lógica é a capacidade de modificar o esquema lógico (O MESMO QUE ESQUEMA CONCEITUAL) sem que, com isso, qualquer programa de aplicação precise ser reescrito. Modificações no nível INTERNO são necessárias, ocasionalmente, para aprimorar o desempenho.”

QUESTÃO CERTA: O SGBD proporciona um conjunto de programas que permite o acesso aos dados sem exposição dos detalhes de representação e armazenamento de dados, por meio de uma visão abstrata dos dados, conhecida como independência de dados.

Em computação, quando falamos de abstração, estamos nos referindo a um conceito que permite relevar os detalhes técnicos de um determinado conceito em favor de uma visão mais ampla, mais próxima do contexto do usuário. Assim, quanto maior o grau de abstração, menos detalhes intrínsecos às maquinas e sistemas são levados em consideração. Nesse caso específico, em outras palavras, o examinador quis dizer que a independência de dados é a característica de abstrair os detalhes da representação e armazenamento dos dados. Não deixa de estar correto, já que o programa de aplicação releva os detalhes da implementação do BD, trazendo assim a possibilidade de se realizar alterações no banco sem precisar alterar o programa

QUESTÃO CERTA: Um esquema de banco de dados geralmente agrupa e apresenta as diferentes tabelas, seus campos e o relacionamento entre eles e outras tabelas.

O termo “esquema de banco de dados” refere-se a uma representação visual de um banco de dados, um conjunto de regras que governa um banco de dados ou todo o conjunto de objetos pertencentes a um determinado usuário.

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Esquemas são apenas descrições dos dados.

Um esquema de banco de dados representa a configuração lógica da totalidade ou de parte de uma base de dados relacional. Ele pode existir tanto como uma representação visual quanto como um conjunto de fórmulas conhecidas como restrições de integridade que regem um banco de dados. Estas fórmulas são expressadas em uma linguagem de definição de dados, como SQL. Como parte de um dicionário de dados, um esquema de banco de dados indica como as entidades que compõem o banco de dados se relacionam entre si, incluindo tabelas, exibições, procedimentos armazenados e muito mais.

QUESTÃO CERTA O objetivo da arquitetura de três esquemas é separar as aplicações do usuário do banco de dados físico. Nessa arquitetura, os esquemas podem ser definidos nos três níveis. A esse respeito, é correto afirmar que o nível externo inclui visões do usuário? Sim. Cada esquema externo descreve a parte do banco de dados em que um grupo de usuários em particular está interessado e oculta o restante do banco de dados do grupo de usuários.

QUESTÃO CERTA: O objetivo da arquitetura em três esquemas é separar a aplicação dos usuários do banco de dados físico. Neste contexto, assinale a alternativa correta: Os três esquemas são chamados de esquema interno, conceitual e externo. Nem todos os sistemas gerenciadores de banco de dados separam completamente os esquemas desta maneira.

QUESTÃO ERRADA: O objetivo da arquitetura em três esquemas é separar a aplicação dos usuários do banco de dados físico. Neste contexto, assinale a alternativa correta: Os três esquemas são chamados de esquema interno, conceitual e externo. Na arquitetura em questão, a independência lógica de dados consiste na capacidade de alterar o “esquema interno” sem mudar o “esquema conceitual” .

A independência lógica de dados é a capacidade de alterar o esquema conceitual sem afetar os esquemas externos ou os programas de aplicativos. A independência de dados físicos é a capacidade de alterar o esquema interno sem afetar o esquema conceitual ou externo.

Independência física de dados

Ao modificar o esquema físico:

Não modifica os esquemas conceitual e, consequentemente, o esquema externo;

Necessidade: aprimoramento do desempenho.

Independência lógica de dados

Ao modificar o esquema conceitual:

Não modifica os programas aplicativos;

Necessidade: alteração da estrutura do BD.

Esquema interno: Descreve detalhes de armazenamento (físico); contém definições de estruturas de dados e mecanismos de acesso.

Esquema conceitual: Descreve a estrutura do BD (entidades, tipos de dados, etc.); definição do conteúdo da informação; utiliza o conceito de modelo de dados; 27 independe de estruturas de dados e mecanismos de acesso;

Esquema externo: Descreve somente a parte do BD pelo qual um grupo de usuários tem interesse – usuário vê apenas parte dos dados; as visões são também chamadas de sub-esquemas.

Objetivos da arquitetura de três níveis:

  • Separar as aplicações dos usuários do BD físico;

  • Prover uma visão abstrata dos dados.

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