删除少于三艘船的所有类请注意，“俾斯麦”级没有船，因此应删除该级

如何解决删除少于三艘船的所有类请注意，“俾斯麦”级没有船，因此应删除该级

[我试图删除所有少于 3 艘船的类，但我无法删除没有船的俾斯麦。我不知道如何删除它有人知道我做错了什么吗？下面的答案是我尝试输入的]

表格

Classes(class,type,country,numGuns,bore,displacement)
Ships(name,class,launched)
Battles(name,date)
Outcomes(ship,battle,result)

我的回答

DELETE FROM classes WHERE class IN 
  (SELECT class FROM ships 
   GROUP BY class HAVING Count(name) < 3) AND NOT EXIST (SELECT * FROM ships);

解决方法

您可以使用 not exists 删除所有 Ships 表中不存在且不少于 3 艘船的类。

delete from Classes
where not exists 
                (
                  select class from Ships group by class having count(*)>=3
                );

,

这应该有效：

DELETE FROM classes WHERE
    (NOT EXISTS (SELECT * from ships s WHERE s.`class` = classes.`class` )) OR
    ((SELECT COUNT(*) from ships s WHERE s.`class` = classes.`class`) < 3)