我不知道它将有多快，但是我想可以这样解决:

I don't know how fast it will be but I guess it can be solved like this:

SELECT ID, min(ORDER_DATE) AS OD,
IF(COUNT(*)=COUNT(CANCEL_DATE),max(CANCEL_DATE),NULL) AS CD 
FROM stats GROUP BY CLIENT

我无法对其进行测试，但是此解决方案背后的想法是count(cancel_date)应该计算所有非空值条目，并且如果它等于count(*)，则意味着没有空值，并且它将返回max(cancel_date) ，否则为null.

I couldn't test it but the idea behind this solution is that count(cancel_date) should count all not null value entries and if it's equal to count(*) that means that there are no null values and it will return max(cancel_date), otherwise null.