O Que É Chave Estrangeira? (conceito, definição e exemplos)

Por
Pedro Santos
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738
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Direção Concursos ensina que: a chave primária da tabela Funcionário é IdFunc (que vai dar uma identificação única para cada funcionário), e a chave estrangeira é IdDep (que faz referência à tabela Departamento, onde IdDep é chave primária).

Uma relação de tabela funciona pela correspondência de dados em campos chave – geralmente um campo com o mesmo nome em ambas as tabelas. Na maioria dos casos, esses campos correspondentes são a chave primária de uma tabela, que fornece um identificador exclusivo para cada registro, e uma chave estrangeira da outra tabela.

É importante que você saiba que uma chave estrangeira não necessariamente referência uma chave primária. A chave estrangeira pode referenciar qualquer atributo ou conjunto de atributos, desde que esses também possam identificar unicamente um registro da tabela de destino.

Ao contrário da chave primária, a chave estrangeira pode sim assumir o valor nulo, desde que o projeto defina que a participação da entidade no relacionamento associado não é total. Por exemplo, se um cliente realizasse uma compra online, esta transação não teria um vendedor associado. Assim, o campo CPFVendedor da venda poderia assumir o valor NULL, indicando que o relacionamento não se aplica àquele registro de venda;

NCE-UFRJ (2009):

QUESTÃO ERRADA: Uma chave estrangeira não pode ter valor NULL.

FADESP (2018):

QUESTÃO CERTA: Sobre os conceitos de banco de dados, é correto afirmar que: é possível ter mais de uma, ou nenhuma, chave estrangeira em uma tabela.

Chave estrangeira (FK) é aquela que faz referência a uma chave primária (PK) ou candidata em outra tabela. É importante ressaltar que essas tabelas pertences ao mesmo banco de dados.

Quando temos uma chave estrangeira em uma tabela, estamos diante de um relacionamento, que são as associações entre as tabelas. A tabela não precisa ter relação com outra tabela, assim como pode ter relacionamento com uma ou mais tabelas, por esse motivo é possível ter mais de uma FK ou nenhuma FK.

FCC (2009):

QUESTÃO CERTA: Uma chave estrangeira de uma tabela relacional é: um atributo que aponta para a chave primária de uma outra tabela.

FCC (2010):

QUESTÃO CERTA: Chaves estrangeiras são os elos de ligação entre as tabelas. Uma coluna definida como chave estrangeira deve ser chave primária em outra tabela.

Correta. Esta é a definição de chave estrangeira. Chaves estrangeiras são os elos de ligação entre as tabelas. Uma coluna definida como chave estrangeira deve ser chave primária em outra tabela.

CEBRASPE (2017):

QUESTÃO ERRADA: A chave estrangeira (foreign key) é o campo que estabelece o relacionamento entre duas tabelas de bancos distintos, sendo necessariamente chave primária na tabela de um dos bancos.

O erro está em dizer que é entre bancos distintos quando na verdade é entre entidades distintas.

Banca própria UFSJ (2009):

QUESTÃO ERRADA: Uma coluna definida como chave-estrangeira em uma tabela-destino não pode aceitar valores nulos e, essa mesma tabela-destino pode ter uma e somente uma coluna definida como chave-estrangeira.

Uma tabela pode ter várias foreign keys, cada uma ligando a uma tabela pai. Chave Estrangeira pode aceitar valores nulos.

COPESE – UFT (2012):

QUESTÃO ERRADA: Chave estrangeira é o atributo que estabelece a relação de uma entidade com a chave secundária de outra entidade e permite uma relação entre entidades.

Chave estrangeira é o atributo que estabelece a relação de uma entidade com a chave primária de outra entidade e permite uma relação entre entidades. Chave estrangeira estabelece relação entre uma entidade com chave primaria e outra entidade, permite referencia-las.

CEBRASPE (2015):

QUESTÃO CERTA: Em um banco de dados relacional, a chave estrangeira que existe em uma tabela deve ser chave primária em outra tabela.

QUESTÃO CERTA: Para que um campo seja FK (chave estrangeira) em uma tabela, é necessário que o mesmo atributo seja PK (chave primária) na tabela de origem.

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