MySQL查询对相同字段分组进行排号

SELECT 
 s.id,s.image_path,s.attach_title,s.attach_required,s.`task_name`,s.originalFilename original, 
 @r:= CASE WHEN @asset_code=s.asset_code THEN @r+1 ELSE 1 END AS rowNum,
 @asset_code:=s.asset_code AS asset_code
 FROM 
 (SELECT i.*, f.attach_title,f.attach_required,m.`task_name`,IFNULL(a.`asset_code`,'') asset_code  
 FROM tfam_asset_checkimage i  
 LEFT JOIN tfam_asset a ON a.id = i.`card_id` 
 LEFT JOIN tfam_asset_checkplan_main_attach f ON f.id = i.`attach_category`  
 LEFT JOIN tfam_asset_checkplan_main m ON f.`main_id` = m.id 
 #WHERE i.main_id = ? AND i.`attach_category` = ? 
 ORDER BY a.`asset_code`) s,(SELECT @r:=0,@asset_code:='') b

按所某字段进行排序，定义变量r序号和某字段初始值，依次赋值，当前值等于上一条记录值则+1否则=1