php - Alias in ZF2 select - with calculated field -

i want create following query zf2:

select (a + b) c ta c > 1; 

with table ta contain 2 integer fields a , b.

i tried far code:

$columns = array('c'=>'(a + b)'); $where = 'c > 1'; $tablegateway = $this->gettablegateway('ta'); $sql = $tablegateway->getsql(); $select = $sql->select()->columns($columns); $select->where($where);     $itemdata = $tablegateway->selectwith($select); 

unfortunately query given is:

select `ta`.`a + b` `c` `ta` c > 1; 

any idea how achieve that? tried without brackets: $columns = array('c'=>'a + b'); not work either.

i've tried before $this->getadapter()->query($sqlquery, adapter::query_mode_execute); have run unbuffered query issue not resolvable ->closecursor().

use zend\db\sql\predicate\expression:

$columns = array('c'=> new expression('(ta.a + ta.b)'));