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Which constraint ensures that a column cannot have duplicate values?

easy📝 Conceptual Q2 of 15
SQL - Table Constraints
Which constraint ensures that a column cannot have duplicate values?
ANOT NULL
BDEFAULT
CCHECK
DUNIQUE
Step-by-Step Solution
Solution:

  1. Step 1: Review constraint types

    UNIQUE prevents duplicate values; NOT NULL prevents empty values; CHECK enforces conditions; DEFAULT sets default values.

  2. Step 2: Match constraint to duplicate prevention

    Only UNIQUE ensures no duplicates in a column.

  3. Final Answer:

    UNIQUE -> Option D

  4. Quick Check:

    Duplicate prevention = UNIQUE [OK]
Quick Trick: UNIQUE stops duplicate entries in a column [OK]
Common Mistakes:
MISTAKES
  • Confusing NOT NULL with UNIQUE
  • Thinking CHECK prevents duplicates
  • Assuming DEFAULT affects duplicates

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