mySql @ variables对不同的客户端的行为不同

I have this query:

我有这个问题:

SELECT @ROWNUM := CASE WHEN @PREV_CAT_ID=T1.category_id THEN @ROWNUM+1 ELSE 1 END AS RANK, 
  @PREV_CAT_ID:=T1.category_id, 
  T1.* 
  FROM(d.category_id
      FROM some_table d,
      (SELECT @ROWNUM := 0)R 
      ORDER BY d.category_id
  }T1 
SELECT @ROWNUM :