Preview:
SELECT 
  GROUP_CONCAT(lv SEPARATOR ',')
  #lv
FROM 
  (
    SELECT 
      @pv :=(
        SELECT 
          GROUP_CONCAT(cid SEPARATOR ',') 
        FROM 
          categories 
        WHERE 
          FIND_IN_SET(parent, @pv)
      ) AS lv 
    FROM 
      categories 
      JOIN (
        SELECT 
          @pv := 5
      ) tmp
  ) tmp2
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