| 31. | "Foreign keys " are integrity constraints enforcing that the operation that draws on information from several relations at once.
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| 32. | The Foreign Key serves as the link, and therefore the connection, between the two related tables in this sample database.
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| 33. | Typically the field being used as the link, the " foreign key ", will be indexed for quick lookup.
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| 34. | For instance, deleting a record that contains a value referred to by a foreign key in another table would break referential integrity.
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| 35. | This means that any one, none, or all, of the multiple attributes within the compound key can be foreign keys.
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| 36. | Foreign keys may be the primary key in another table; for example a PersonID may become the EmployeeID in the Employee table.
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| 37. | Both foreign keys and superkeys ( that includes candidate keys ) can be composite, that is, can be composed of several attributes.
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| 38. | In other words, when a foreign key value is used it must reference a valid, existing primary key in the parent table.
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| 39. | The adjective'referential'describes the action that a foreign key performs,'referring'to a link column in another table.
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| 40. | In simpler words, the foreign key is defined in a second table, but it refers to the primary key in the first table.
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