如何筛选SQL中出现值的最大计数？

我有以下疑问：

SELECT customer_id, product_id, COUNT(product_id) CUSTOMER_PRODUCT_COUNT
FROM Order_Details
Group BY customer_id, product_id
ORDER BY customer_id;

其结果如下表所示：

    CUSTOMER_I PRODUCT_ID CUSTOMER_PRODUCT_COUNT
---------- ---------- ----------------------
C1         P3                              2
C1         P4                              1
C2         P5                              1
C3         P2                              4
C3         P3                              1
C3         P4                              1
C4         P1                              3
C4         P6                              1
C5         P6                              1

9 rows selected.

我在筛选出每个客户的产品id的最大出现次数时遇到问题

CUSTOMER_ID PRODUCT_ID CUSTOMER_PRODUCT_COUNT
---------- ---------- ----------------------
C1         P3                              2
C2         P5                              1
C3         P2                              4
C4         P1                              3
C5         P6                              1

---

对于除C1和C3之外的客户id，计数1的出现可以省略，也可以不省略。只有您可以使用

行编号（）

窗口功能为每个记录分配一个编号，该编号根据每个客户的计数指定记录的顺序，然后根据该编号进行过滤

SELECT customer_id,
       product_id,
       customer_product_count
       FROM (SELECT customer_id,
                    product_id,
                    count(product_id) customer_product_count,
                    row_number() OVER (PARTITION BY customer_id
                                       ORDER BY count(product_id) DESC) r
                    FROM order_details
                    GROUP BY customer_id,
                             product_id
                    ORDER BY customer_id) x
       WHERE r = 1;

---

如果要包括关系，请使用

rank（）

而不是

row\u number（）

Oracle中的另一种方法是两级聚合：

SELECT customer_id, CUSTOMER_PRODUCT_COUNT,
       MAX(product_id) KEEP (DENSE_RANK FIRST ORDER BY CUSTOMER_PRODUCT_COUNT DESC) as product_id
FROM (SELECT customer_id, product_id, COUNT(*) AS CUSTOMER_PRODUCT_COUNT
      FROM Order_Details
      GROUP BY customer_id, product_id
     ) cp
GROUP BY customer_id
ORDER BY customer_id;

---

KEEP

表达式是Oracle定义“first（）”聚合函数的方法。

这正是答案所在