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I wonder if I can put a foreign key in more than one table, for example, where the entity "competitors" in the image is a table and the attribute "id_competitor" is a primary key, and the other entities "competitors" document, "competitors", "competitions_contact" and "competitions_placement" have as a foreign key the "id_competitor", would it be possible to do so? If not what better option to do?
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@Lone Tomberry, not only can, but should. Foreign keys or Foreign key, as they are commonly called ensure the integrity of a database data. Note the type of relationship between your tables. The following are two reference articles.
The table can also have as attribute only a primary key ?
Yes. But a primary key can be formed by more than one column, so it becomes one composite primary key. Read one about it at this link: http://www.luis.blog.br/chave-primaria-simples-e-chave-primaria-composta.aspx
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Each of them should have a FK (Foreign Key) related to the competitor. There’s nothing wrong with that.
– Gustavo Cinque