ZF2 Sql Exists sub query using ZF2 query helpers

original.sql

SELECT *
FROM SOME_TABLE_ONE STO
WHERE EXISTS (
  SELECT *
  FROM SOME_TABLE_TWO STT
  WHERE STO.STT_ID = STT.ID
)

query.php

<?PHP

use Zend\Db\Sql\Sql;
use Zend\Db\Sql\Predicate\Expression;

public function getData() {

  // Get the DB adapter
  $adapter = $this->getAdapter()
  
  // Build the main query
  $sql = new Sql($adapter);
  $select = $sql->select();
  $select->from(array('STO' => 'SOME_TABLE_ONE'));
  
  // Create subquery
  $subSelect = $sql->select();
  $subSelect->from(array('STT' => 'SOME_TABLE_TWO'));
  $subSelect->where('STO.STT_ID = STT.ID');
  
  // Add the subquery as a predicate to the main query
  /*
   * Notice the @ symbol to suppress errors on the getSqlString() method.
   * See: http://framework.zend.com/security/advisory/ZF2013-03
   */
  $select->where->addPredicate(
    new Expression(
      "EXISTS(" . @$subSelect->getSqlString($adapter->getPlatform()) . ")"
    )
  );
  
  // Run the query
  $statement = $sql->prepareStatementForSqlObject($select);
  $data = $statement->execute();
}