如何在postgresql中找到所有其他产品的平均值

Question

我有一张表Products，如下所示：

+-----------+-----------+----------+
|ProductCode|ProductType|   ....   |
+-----------+-----------+----------+
|   ref01   |   BOOKS   |   ....   |
|   ref02   |   ALBUMS  |   ....   |
|   ref06   |   BOOKS   |   ....   |
|   ref04   |   BOOKS   |   ....   |
|   ref07   |   ALBUMS  |   ....   |
|   ref10   |   TOYS    |   ....   |
|   ref13   |   TOYS    |   ....   |
|   ref09   |   ALBUMS  |   ....   |
|   ref29   |   TOYS    |   ....   |
|   .....   |   .....   |   ....   |
+-----------+-----------+----------+

另一个表Sales，如下所示：

+-----------+-----------+----------+
|ProductCode|   Orders  |   ....   |
+-----------+-----------+----------+
|   ref01   |     15    |   ....   |
|   ref02   |     12    |   ....   |
|   ref06   |     20    |   ....   |
|   ref04   |     14    |   ....   |
|   ref07   |     11    |   ....   |
|   ref10   |     19    |   ....   |
|   ref13   |      3    |   ....   |
|   ref09   |      9    |   ....   |
|   ref29   |      5    |   ....   |
|   .....   |   .....   |   ....   |
+-----------+-----------+----------+

我正在尝试查找订购量超过所有其他同类产品的平均值的产品。

通过手动计算，结果会是这样的：

+-----------+-----------+----------+
|ProductCode|   Orders  |   ....   |
+-----------+-----------+----------+
|   ref02   |     12    |   ....   |
|   ref06   |     20    |   ....   |
|   ref07   |     11    |   ....   |
|   ref10   |     19    |   ....   |
|   .....   |   .....   |   ....   |
+-----------+-----------+----------+

因此，如果查看类型 ALBUMS 和产品 ref02，那么我需要找到所有其他 ALBUMS 的订单的平均值。 在这种情况下，它是ref06 和ref04 的平均值，但实际表中还有更多。所以我需要做的是：

Since product ref02 is 'ALBUMS', and ref07 and ref09 are also 'ALBUMS'. 
      So their average is (11+9)/2=10 <12.

Since product ref06 is 'BOOKS', and **ref01** and ref04 are also 'BOOKS'.                     
      So their average is (15+14)/2=14.5 <20.

Since product ref07 is 'ALBUMS', and **ref02** and ref09 are also 'ALBUMS'.           
      So their average is (12+9)/2=10.5<11.

Since product ref10 is 'TOYS', and ref13 and ref29 are also 'TOYS'           
      So their average is (3+5)/2=4<19.

The rest does not satisfy the condition thus will not be in the result.

我知道如何并且能够找到同一类型下所有产品的订单平均值，但我不知道如何找到同一类型下所有其他产品的订单平均值。

我正在使用 PostgreSQL，但不能使用以下任何关键字：WITH、OVER、LIMIT、PARTITION。

Answer 1

这是我将在 Postgres 11 或更高版本中使用的内容：
使用自定义框架定义的窗口函数的解决方案：

SELECT product_code, orders
FROM  (
   SELECT product_code, s.orders
        , avg(orders) OVER (PARTITION BY p.product_type
                            ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
                            EXCLUDE CURRENT ROW) AS avg_orders
   FROM   product p
   JOIN   sales s USING (product_code)
   ) sub
WHERE  orders > avg_orders 
ORDER  BY product_code;  -- optional

我希望它表现最好。
阅读the manual 了解窗口函数的详细信息。

这就是您要的：
没有 CTE、窗口函数或 LIMIT 的解决方案（适用于任何现代 Postgres 版本）：

SELECT product_code, s.orders
FROM   product p
JOIN   sales s USING (product_code)
JOIN   LATERAL (
   SELECT avg(orders) AS avg_orders
   FROM   product p1
   JOIN   sales s1 USING (product_code)
   WHERE  p1.product_type =  p.product_type
   AND    p1.product_code <> p.product_code
   ) a ON a.avg_orders < s.orders
ORDER  BY product_code;  -- optional

两者都能产生你想要的结果。

db小提琴here

关于LATERAL：

• What is the difference between LATERAL JOIN and a subquery in PostgreSQL?

Answer 2

我们可以在这里尝试使用AVG()作为解析函数：

WITH cte AS (
    SELECT p.ProductCode, p.ProductType, s.Orders,
           AVG(s.Orders) OVER (PARTITION BY p.ProductType) AS AvgOrders
    FROM Products p
    INNER JOIN Sales s ON s.ProductCode = p.ProductCode
)

SELECT ProductCode, ProductType, Orders
FROM cte
WHERE Orders > AvgOrders;

Demo