Question

早上好。 SQL新手在这里寻求帮助。我正在尝试从子查询中获取结果集的AVG。子查询可以正常工作，并为我提供所需的结果集，但我无法使结果集的AVG正常工作。任何指导将不胜感激。

SELECT AVG(COUNT) FROM   
(SELECT COUNT(DISTINCT(table2.item_no))   
FROM table1   
JOIN table2 ON table1.order_numb = table2.order_numb
WHERE user_so = 'paul'
AND order_date BETWEEN '9/20/2017'AND '9/20/2018'
GROUP BY table1.order_numb);

这里是我自己试图从子查询中得到的结果集的一个样本，然后我尝试转而得到以下AVG：

Answer 1

我认为您想要这样的东西：

SELECT AVG(a_number) FROM   
(SELECT COUNT(DISTINCT(table2.item_no)) AS a_number  
FROM table1   
JOIN table2 ON table1.order_numb = table2.order_numb
WHERE user_so = 'paul'
AND order_date BETWEEN '9/20/2017'AND '9/20/2018'
GROUP BY table1.order_numb) AS the_subquery

我没有相同的数据表，所以这是一个简化的示例：

SELECT AVG(a_number) FROM (
SELECT 100 AS a_number
UNION
SELECT 200 AS a_number
UNION
SELECT 300 AS a_number
UNION
SELECT 400 AS a_number
) AS the_subquery

您似乎缺少需要为子查询加上别名，并且想AVG为子查询中返回的值的别名。

更新

根据评论，如果您希望答案四舍五入到小数点后两位，则需要CAST更改为其他数据类型，如下所示：

SELECT ROUND(AVG(CAST(a_number AS FLOAT)), 2) FROM   
(SELECT COUNT(DISTINCT(table2.item_no)) AS a_number  
FROM table1   
JOIN table2 ON table1.order_numb = table2.order_numb
WHERE user_so = 'paul'
AND order_date BETWEEN '9/20/2017'AND '9/20/2018'
GROUP BY table1.order_numb) AS the_subquery

或其他无法访问该表的人

SELECT ROUND(AVG(CAST(a_number AS FLOAT)), 2) FROM (
SELECT 100 AS a_number
UNION
SELECT 200 AS a_number
UNION
SELECT 300 AS a_number
UNION
SELECT 403 AS a_number
) AS the_subquery

您可以在此处详细了解ROUND，AVG和CAST的工作方式：How do I retrieve decimals when rounding an average in SQL

结果集的AVG

1 个答案: