使用union是zf2：
使用zf2专用类 Combine zend\db\sql\combine组合

new Combine(
     [
      $select1,
      $select2,
      $select3,
       ...
     ]
    )

使用合并的详细示例如下：

$select1 = $sql->select('java');
$select2 = $sql->select('dotnet');
$select1->combine($select2);

$select3 = $sql->select('android');

$selectall3 = $sql->select();
$selectall3->from(array('sel1and2' => $select1));
$selectall3->combine($select3);

$select4 = $sql->select('network');

$selectall4 = $sql->select();
$selectall4->from(array('sel1and2and3' => $selectall3));
$selectall4->combine($select4);

$select5 = $sql->select('dmining');

$selectall5 = $sql->select();
$selectall5->from(array('sel1and2and3and4' => $selectall4));
$selectall5->combine($select5);

相当于union的常规sql查询：

SELECT * FROM java 
UNION SELECT * from dotnet 
UNION SELECT * from android 
UNION SELECT * from network;
UNION SELECT * from dmining;

希望对你有帮助。