如果你有样本数据：

CREATE TABLE purchase ( purchasedate ) AS
SELECT DATE '2019-01-01' + LEVEL - 1 FROM DUAL CONNECT BY LEVEL <= 5 UNION ALL
SELECT DATE '2019-02-01' + LEVEL - 1 FROM DUAL CONNECT BY LEVEL <= 3 UNION ALL
SELECT DATE '2020-01-01' + LEVEL - 1 FROM DUAL CONNECT BY LEVEL <= 3 UNION ALL
SELECT DATE '2020-02-01' + LEVEL - 1 FROM DUAL CONNECT BY LEVEL <= 2 UNION ALL
SELECT DATE '2020-03-01' + LEVEL - 1 FROM DUAL CONNECT BY LEVEL <= 4 UNION ALL
SELECT DATE '2020-04-01' + LEVEL - 1 FROM DUAL CONNECT BY LEVEL <= 1;

然后，你可以使用查询：

SELECT TO_CHAR(PURCHASEDATE, 'MM YYYY') AS monthyear,
       COUNT(*) AS frequency
FROM   PURCHASE
GROUP BY
       TO_CHAR(PURCHASEDATE, 'MM YYYY');

哪个输出：

MONTHYEAR | 频率
：-------- | --------：
2020年3月| 4
2019年1月| 5
2020年1月| 3
2020年2月| 2个
2019年2月| 3
2020年4月| 1个

或者，你可以使用TRUNC：

SELECT TRUNC(PURCHASEDATE,'MM') AS monthyear,
       COUNT(*) AS frequency
FROM   PURCHASE
GROUP BY
       TRUNC(PURCHASEDATE,'MM');

哪个输出：

MONTHYEAR | 频率
：------------------ | --------：
2020-03-01 00:00:00 | 4
2020-04-01 00:00:00 | 1个
2020-02-01 00:00:00 | 2个
2020-01-01 00:00:00 | 3
2019-01-01 00:00:00 | 5
2019-02-01 00:00:00 | 3

或者，你可以使用EXTRACT：

SELECT EXTRACT( YEAR FROM PURCHASEDATE) AS year,
       EXTRACT( MONTH FROM PURCHASEDATE) AS month,
       COUNT(*) AS frequency
FROM   PURCHASE
GROUP BY
       EXTRACT( YEAR FROM PURCHASEDATE),
       EXTRACT( MONTH FROM PURCHASEDATE);

哪个输出：

年| MONTH | 频率
---：| ----：| --------：
2019 | 1 | 5
2020 | 1 | 3
2020 | 2 | 2个
2020 | 4 | 1个
2019 | 2 | 3
2020 | 3 | 4

db <>在这里拨弄