Resumo das páginas 13 – 31
O grupo ANSI/X3/SPARC se dedicou um tempo considerável para obter diferentes visões da teoria de banco de dados como também para desenvolver um vocabulário que fosse conciso e entendido por todos . Eles apresentaram uma proposta que possuía uma estrutura repartida em três níveis : o esquema externo (esquema do programador), o esquema interno (representado pela organização física dos dados nos dispositivos de armazenamento ) e o esquema conceitual ( representado pela visão da empresa em relação aos dados).
O grupo ANSI/X3/SPARC identificou três categorias de administradores de banco de dados : Administrador de empresa que define o esquema conceitual , Administrador de banco de dados que define o esquema interno relacionado a parte de armazenamento e Administrador da aplicação que representa uma visão dos dados pelo programador da aplicação . Esses três níveis de abstração foram definidos para fornecer uma maneira organizada de projetar e gerenciar sistemas de banco de dados, permitindo uma clara separação entre a visão do usuário, a estrutura lógica e a implementação física.
o conceito de “entidade” se refere-se a um objeto ou conceito do mundo real que pode ser identificado e distingui-los de outros objetos. Relacionamentos podem existir entre entidades . Eles representam as associações e interações entre diferentes entidades, ajudando a refletir as relações do mundo real que estão sendo modeladas. Existem diferentes tipos de relacionamentos entre entidades e alguns deles são de interesse para a empresa . O projetista do banco de dados é responsável por especificar os tipos de mapeamento dos tipos de relacionamentos (um-para-um , um-para-muitos ,muitos-para-muitos).
Entidades e relacionamentos possuem propriedades que podem ser definidas em termos de atributo-valor . Atributos definem as características de uma entidade, e valores são as instâncias específicas desses atributos associados a uma entidade particular. A combinação de atributos e valores ajuda a representar e organizar as informações no contexto de um banco de dados.
A existência de uma entidade pode depender da existência de uma outra . Ou seja a existência de uma entidade fraca depende da existência de outras entidades . No relacionamento um-para-muitos : Uma instância de uma entidade na extremidade “um” está associada a várias instâncias da outra entidade na extremidade “muitos . No relacionamento muitos-para-muitos : Várias instâncias em ambas as entidades podem estar associadas umas às outras. Em uma tabela, o identificador (ou chave primária) é o atributo (ou conjunto de atributos) que identifica de forma exclusiva cada registro na tabela.
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