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      Question

      Which of the following correctly describes a 'primary

      key' in a relational database?
      A A key that can have NULL values Correct Answer Incorrect Answer
      B A key that references a column in another table Correct Answer Incorrect Answer
      C A key that uniquely identifies each record in a table and cannot be NULL Correct Answer Incorrect Answer
      D A key used only for indexing purposes Correct Answer Incorrect Answer
      E A key that can have duplicate values if combined with another column Correct Answer Incorrect Answer

      Solution

      A primary key enforces entity integrity — it must be unique for each row and cannot contain NULL values. A foreign key references a primary key in another table (referential integrity). A composite key uses multiple columns together. A candidate key is any column that could serve as a primary key.

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