php – 更新列,使其包含行位置

这是内容表：

ContentID | CategoryID | Position | Other1 | Other2
===================================================
1         | 1          | NULL     | abcd   | efgh
2         | 1          | NULL     | abcd   | efgh
3         | 1          | NULL     | abcd   | efgh
4         | 2          | NULL     | abcd   | efgh
5         | 2          | NULL     | abcd   | efgh
6         | 2          | NULL     | abcd   | efgh

这些是我将要运行的查询：

SELECT ContentID FROM content WHERE CategoryID = 1 ORDER BY Position
SELECT ContentID FROM content WHERE CategoryID = 2 ORDER BY Position

现在我想实现向上移动,向下移动,向上移动到底部功能的内容.我需要做的就是用数字填充Position列：

ContentID | CategoryID | Position
=================================
1         | 1          | 1
2         | 1          | 2
3         | 1          | 3
4         | 2          | 1
5         | 2          | 2
6         | 2          | 3

是否可以通过MysqL中的单个查询来实现？就像是：

UPDATE content
SET Position = <ROW_NUMBER>
WHERE CategoryID = 1
ORDER BY Position

UPDATE content
SET Position = <ROW_NUMBER>
WHERE CategoryID = 2
ORDER BY Position

这应该工作

update 
content,(
  select 
  @row_number:=ifnull(@row_number,0)+1 as new_position,ContentID 
  from content
  where CategoryID=1
  order by position
) as table_position
set position=table_position.new_position
where table_position.ContentID=content.ContentID;

但是我首先应用这个来取消设置用户定义的变量

set @row_number:=0;

由Mchl添加：

你可以在这样的一个声明中做到这一点

update 
content,ContentID 
  from content
  where CategoryID=1
  order by position
) as table_position,(
  select @row_number:=0
) as rowNumberInit
set position=table_position.new_position
where table_position.ContentID=content.ContentID;