$field = 'id,title,cover';
$isql = Db::name('a')->field($field)->where('title|description', 'like', "%$keyword%")->buildSql();
$tsql = Db::name('b')->field($field)->where('title|description', 'like', "%$keyword%")->buildSql();
$sql = Db::name('c')->field($field)->where('title|description', 'like', "%$keyword%")->union([$isql, $tsql])->buildSql();
$r = Db::table($sql.' b')->order('view_num desc')->paginate($this->pagelist);
如果给sql加标识可以写为$isql = Db::name('a')->field($field.',1 as type')->where('title|description', 'like', "%$keyword%")->buildSql();