thinkphp 5 union 聯合查詢

$a = Db::name(表名)->field(字段)->where(條件)->buildSql();
$b = Db::name(表名)->field(字段)->where(條件)->buildSql();
$c = Db::name(表名)->field(字段)->where(條件)->union([$a, $b])->buildSql();
$list = Db::table($c . ' a')->order(排序)->paginate(每頁個數);// $c . a是零時表名