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Is there any different how mysql treats

UNIQUE (column1, column2),
INDEX (column1)

and

UNIQUE INDEX (column1, column2)

and

UNIQUE (column1, column2)

In the latter, I think mysql will automatically creates index for UNIQUE to avoid duplicate records. Am I right?

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1 Answer 1

up vote 7 down vote accepted

UNIQUE (column1, column2) implies UNIQUE INDEX (column1, column2) because the INDEX keyword is optional. So an index is created. However, the MySQL 5.5 docs show that the INDEX (or KEY) keyword is mandatory so UNIQUE (column1, column2) should give an error

INDEX (column1, column2) does not mean UNIQUE INDEX (column1, column2): it means an index that does allow duplicate (column1, column2) pairs

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+1 for quoting the change in mysql 5.5! –  All Mar 26 '12 at 13:11
1  
@gbn: The linked page has this: [CONSTRAINT [symbol]] UNIQUE [INDEX|KEY], so UNIQUE (column1, column2) does not give an error. Neither in 5.1, nor in 5.5. –  ypercube Mar 26 '12 at 13:31
    
@ypercube: oh yeah, read the docs wrong! –  gbn Mar 26 '12 at 13:32

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