How to use Distinct in magento?

Use group by instead of distinct.

Add $collection->getSelect()->group('level').

Use group by instead of ->distinct(true).

Code:

$collection = Mage::getModel('school/student')->getCollection()
          ->addFieldToSelect('level');

$collection->getSelect()->group('level')

Or use:

$collection = Mage::getModel('school/student')->getCollection()->getSelect()
      ->columns('level')
      ->group('level');

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