MySQL希望所有结果都来自左表,并且只过滤带有条件的右表

您需要将条件移动到 on 条款

select T.t_id as TicektID, TK.tk_category as Category,
       coalesce(T.contact_person, 'N/A') as Contact_Person,
       T.comment as Employee_Comment,
       COUNT(TD.t_detail_id) as num_comments, 
       T.t_status
from sun_it_tickets T left join
     sun_it_ticket_categories tk
     on TK.tk_id = T.tk_id left join
     sun_it_tickets_detail TD 
     on TD.t_id = T.t_id and TD.tech_viewed = 0
group by Category, Contact_Person, Employee_Comment, t_status
order by max(T.submit_time);

笔记:

• 不要对列别名使用单引号。仅对字符串和日期常量使用单引号。
• 所有非聚合列都应位于 group by .
• 这个 order by 应该在
• coalesce() 是ISO/ANSI标准函数,因此我更喜欢使用它,而不是数据库特定的替代方法。