A uniqueness constraint ensures that all values in a column are different from one another. This prevents duplicate entries that could lead to inconsistencies in the database. For example, if the uniqueness constraint is applied to a customer email column, it guarantees that each email address is entered only once, thus maintaining the uniqueness of customer records. The other options are incorrect because a foreign key connects tables together, a primary key both uniquely identifies each row and disallows NULLs (making it more restrictive than a simple uniqueness constraint), and ensuring that every record relates to another record refers to referential integrity, not uniqueness.
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What is the difference between a uniqueness constraint and a primary key?
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How does a uniqueness constraint prevent duplicate entries in a database?
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Can a uniqueness constraint be applied to multiple columns at once?