/**
* Render DISTINCT clause
*
* @param string $sql SQL query
* @return string|null
*/
protected function _renderColumns($sql)
{
if (!count($this->_parts[self::COLUMNS])) {
return null;
}
$columns = array();
foreach ($this->_parts[self::COLUMNS] as $columnEntry) {
list($correlationName, $column, $alias) = $columnEntry;
if ($column instanceof Expr) {
$columns[] = $this->_adapter->quoteColumnAs($column, $alias, true);
} else {
if ($column == self::SQL_WILDCARD) {
$column = new Expr(self::SQL_WILDCARD);
$alias = null;
}
if (empty($correlationName)) {
$columns[] = $this->_adapter->quoteColumnAs($column, $alias, true);
} else {
$columns[] = $this->_adapter->quoteColumnAs(array($correlationName, $column), $alias, true);
}
}
}
return $sql .= ' ' . implode(', ', $columns);
}