Oracle-将行中的值转换为范围

我觉得可能可以对此进行改进，但是它可以起作用：

WITH base AS  (    SELECt  1 N FROM DUAL UNIOn ALL    SELECt  2 N FROM DUAL UNIOn ALL    SELECt  3 N FROM DUAL UNIOn ALL    SELECt  6 N FROM DUAL UNIOn ALL    SELECt  7 N FROM DUAL UNIOn ALL    SELECt 17 N FROM DUAL UNIOn ALL    SELECt 18 N FROM DUAL UNIOn ALL    SELECt 19 N FROM DUAL UNIOn ALL    SELECt 21 N FROM DUAL), lagged AS(    SELECt n, LAG(n) OVER (ORDER BY n) lag_n FROM base), groups AS(    SELECt n, row_number() OVER (ORDER BY n) groupnum      FROM lagged      WHERe lag_n IS NULL OR lag_n < n-1), grouped AS(    SELECt n, (SELECT MAX(groupnum) FROM groups      WHERe groups.n <= base.n   ) groupnum      FROM base)SELECt groupnum, MIN(n), MAX(n)  FROM grouped  GROUP BY groupnum  ORDER BY groupnum