ROW_NUMBER结合 count使用

可以利用ROW_NUMBER函数和COUNT函数相结合实现分组排序并计算每组的行数。例如，对于一个表格orders，可以按照customer_id进行分组排序，并计算每个customer_id对应的订单数量和每个订单在该组中的排名，可以使用以下SQL语句：

SELECT customer_id, COUNT(*) as order_count, ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY order_date DESC) as row_num
FROM orders
GROUP BY customer_id

其中，ROW_NUMBER函数将每个分组内的订单按照order_date降序排列，并给每个订单分配一个排名。PARTITION BY子句指定了按照customer_id分组，ORDER BY子句指定了按照order_date降序排列。COUNT函数计算每个分组内的订单数量，并将其命名为order_count。最终的结果包含三列：customer_id、order_count和row_num。

相关问题

求每科前十名的学生用row_number和不用row_number的写法

好的，我来回答你的问题。

使用 row_number 的写法：

SELECT subject, name, score
FROM (
  SELECT subject, name, score, ROW_NUMBER() OVER (PARTITION BY subject ORDER BY score DESC) AS rank
  FROM scores
) AS t
WHERE rank <= 10;

不使用 row_number 的写法：

SELECT s1.subject, s1.name, s1.score
FROM scores s1
WHERE (
  SELECT COUNT(*)
  FROM scores s2
  WHERE s2.subject = s1.subject AND s2.score > s1.score
) < 10;

以上两种写法都可以得到每科前十名的学生。

优化一下我的sql语句：WITH t1 AS ( SELECT *, row_number() OVER (PARTITION BY user_id) rn FROM web_operation ) SELECT log_time,count(1) cnt FROM t1 WHERE rn =1 GROUP BY log_time;

我可以尝试优化你的SQL语句：SELECT log_time, COUNT(1) cnt FROM web_operation WHERE user_id IN (SELECT user_id FROM web_operation GROUP BY user_id) GROUP BY log_time;