Complex query in Cakephp 3

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Hello, how can I mount this query with Cakephp 3?

SELECT DISTINCT campo FROM (
         SELECT `var1` campo FROM tabla WHERE `var1` is not null UNION ALL
         SELECT `var2` campo FROM tabla WHERE `var2` is not null ) T ORDER BY campo

I think it’s something like this:

$var1 = TableRegistry::getTableLocator()->get('Tabla')
                                                                 ->select(['Tabla.var1'])
                                                                 ->where(['Tabla.var1 is not null']);
$var2 = TableRegistry::getTableLocator()->get('Tabla')
                                                                ->select(['Tabla.var2'])
                                                               ->where(['Tabla.var2 is not null']);
$unir = $var1->unionAll($var2);

$campo = TableRegistry::getTableLocator()->get('Tabla')->find()
                                                                     ->distinct()
                                                                     ->from([Tabla->alias() => $unir])
                                                                     ->order(['Tabla.campo' => 'DESC']);

It is a single table and combines two fields into one, sorts them and removes repeating or null records.