ThinkPHP group by自定义规则分组

SELECT 
CASE WHEN `type` IN('A','C','D') THEN 'ACD'
WHEN `type` IN ('F', 'G') THEN 'FG'
ELSE `type` END AS `types`, SUM(num)
FROM demo
GROUP BY `types` 

等同于：
SELECT 
CASE WHEN `type` IN('A','C','D') THEN 'ACD'
WHEN `type` IN ('F', 'G') THEN 'FG'
ELSE `type` END AS `types`, SUM(num)
FROM demo
GROUP BY 
CASE WHEN `type` IN('A','C','D') THEN 'ACD'
WHEN `type` IN ('F', 'G') THEN 'FG'
ELSE `type` END

$list = Db::name('Demo')
    ->field("SUM(num),CASE WHEN `type` IN('A','C','D') THEN 'ACD'
        WHEN `type` IN ('F', 'G') THEN 'FG'
        ELSE `type` END AS `types`")
    ->order('id DESC')->select();