使用CASE表达式：

SELECT CASE WHEN dif IN ('c', 'd') THEN 'cd' ELSE dif END AS dif,
       COUNT(*)
FROM   table_name
GROUP BY
       CASE WHEN dif IN ('c', 'd') THEN 'cd' ELSE dif END;

其中，对于样本数据：

CREATE TABLE table_name (dif) AS
SELECT 'a' FROM DUAL CONNECT BY LEVEL <= 5 UNION ALL
SELECT 'b' FROM DUAL CONNECT BY LEVEL <= 3 UNION ALL
SELECT 'c' FROM DUAL CONNECT BY LEVEL <= 6 UNION ALL
SELECT 'd' FROM DUAL CONNECT BY LEVEL <= 7;

输出：
| DIF| COUNT（*）|
| - -----|- -----|
| B| 3|
| 一种|5个|
| 光盘|十三个|
fiddle

Case应该可以

select col5,count(*) from TABLE1_ GROUP BY col5

返回

使用Case定义一个新的变量来分组，如下所示：

SELECT 
  CASE
    WHEN col5 IN ('brown', 'green') THEN 'browngreen'
    ELSE col5
  END AS GroupedDIF,
  COUNT(*) AS Count_
FROM TABLE1_
GROUP BY GroupedDIF;

返回

注意：这是用postgresql而不是oracle完成的，但应该是可以理解的