Improve grammatical consistency (#774)
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@ -106,7 +106,7 @@ There are two types of keys:
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each plane in the `planes` table.
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each plane in the `planes` table.
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* A __foreign key__ uniquely identifies an observation in another table.
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* A __foreign key__ uniquely identifies an observation in another table.
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For example, the `flights$tailnum` is a foreign key because it appears in the
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For example, `flights$tailnum` is a foreign key because it appears in the
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`flights` table where it matches each flight to a unique plane.
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`flights` table where it matches each flight to a unique plane.
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A variable can be both a primary key _and_ a foreign key. For example, `origin` is part of the `weather` primary key, and is also a foreign key for the `airport` table.
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A variable can be both a primary key _and_ a foreign key. For example, `origin` is part of the `weather` primary key, and is also a foreign key for the `airport` table.
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