**Which key cannot be null** - (i) Unique key - (ii) Primary key - (iii) Super key - (iv) Foreign key

Which key cannot be null - (i) Unique key - (ii) Primary key - (iii) S

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Unit 1: Database System Architecture

20237m

Which key cannot be null

(i) Unique key
(ii) Primary key
(iii) Super key
(iv) Foreign key

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Answer: (ii) Primary key

Explanation: Primary key cannot be NULL and must be unique - entity integrity constraint. Unique key can have one NULL (in many DBMS) but cannot duplicate. Super key is any set of attributes that uniquely identifies row, may include extra attributes. Foreign key can be NULL (if relationship optional) - referential integrity allows NULL.

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