Db::getQueryBuilder() 

\nn\t3::Db()->getQueryBuilder($table = ''); 

Get QueryBuilder for a table

$queryBuilder = \nn\t3::Db()->getQueryBuilder( 'fe_users' );
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Example:

$queryBuilder = \nn\t3::Db()->getQueryBuilder( 'fe_users' );
$queryBuilder->select('name')->from( 'fe_users' );
$queryBuilder->andWhere( $queryBuilder->expr()->eq( 'uid', $queryBuilder->createNamedParameter(12) ));
$rows = $queryBuilder->executeStatement()->fetchAllAssociative();
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@param string $table
@return QueryBuilder

Source Code 

public function getQueryBuilder( $table = '' )
{
	$queryBuilder = GeneralUtility::makeInstance(ConnectionPool::class)->getQueryBuilderForTable( $table );
	return $queryBuilder;
}
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