mysql - Setting order/position - resetting by two vars -

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I want to ask that it can only be done by MySQL and not by php code

my I have a table that contains 

  id, catid, order   
 I know that  
  SET @ pos: = 0; Update products SET order = @pos: = @ pin 1 order order;   
 But how do I reset it (starting at 0 on each cadet).  
 I can choose each cat in php and can run different SQL each time.      
 
   
 "post-text" itemprop = "text"> 
 Introduce a new variable @cat, compare with Catid and reset @pos on the cached changes  
  SET @pos: = 0; SET @cat: = 0; Enter Product P (Choose ID, @pos: = if (cat cat> 0, @ pos + 1) as new_order, order by product ORDER from @cat: = catid) p2 p Id = p2 Id set order = p2.new_order;

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