条件に応じてdbから時間を取得する方法

timestampdiff を使用します そのための関数とCASE ：

select case 
        when TIMESTAMPDIFF(SECOND, `date`,current_timestamp) <= 60 
        then concat(TIMESTAMPDIFF(SECOND, `date`,current_timestamp), ' seconds')
        when TIMESTAMPDIFF(DAY, `date`,current_timestamp) <= 3 
        and TIMESTAMPDIFF(DAY, `date`,current_timestamp) >= 1 
        ...
       end as time_diff 
from Table_hello
where TIMESTAMPDIFF(DAY, `time`,current_timestamp) >= 3