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Which constraint allows a column to contain NULL values but does not allow duplicate non-NULL values (in most DBMSs)?

1546Medium DSSSB TGT Computer Science 0 replies 1 views
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Which constraint allows a column to contain NULL values but does not allow duplicate non-NULL values (in most DBMSs)?

  • APRIMARY KEY
  • BUNIQUE
  • CCHECK
  • DFOREIGN KEY
Correct answer: B

Explanation

UNIQUE rejects duplicate non-NULL values but permits NULLs, whereas PRIMARY KEY forbids NULLs.
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