MySQL查询返回null

Question

select sum(c.cost)
from (select x.num as x, y.num as y, max(priority) as maxpriority
      from numbers x cross join
           numbers y join
           costs c
           on x.num between c.x and c.x + c.deltax and y.num between c.y and c.y + c.deltay
      where x.num between PIXELX and PIXELX and DELTAX and
            y.num between PIXELY and PIXELY and DELTAY
      group by x.num, y.num
    ) xyp join
    costs c
    on xyp.x between c.x and c.x + c.deltax and xyp.y between c.y + c.deltay and
        xyp.maxpriority = c.priority

我试图获得2d中所选区域的“成本”，该区域由不同的区域大小填充，不同的成本按优先级相互重叠。 （如果一个区域低于另一个区域，则忽略它）

这是数字：

Collation   Attributes  Null    Default Extra   Action
     1  num int(11)         No  None          Change      Drop    Browse distinct values     More

这是费用：

#   Name    Type    Collation   Attributes  Null    Default Extra   Action
 1  x   int(11)         No  None          Change      Drop    Browse distinct values     More
 2  y   int(11)         No  None          Change      Drop    Browse distinct values     More
 3  deltax  int(11)         No  None          Change      Drop    Browse distinct values     More
 4  deltay  int(11)         No  None          Change      Drop    Browse distinct values     More
 5  priority    int(11)         No  None          Change      Drop    Browse distinct values     More

我错过了什么？代码可能存在错误。