如何在 BETWEEN 子句中插入 AVG？

Question

以 Northwind 为例。我想使用 CASE 子句通过比较 units_in_stock 与其 AVG 值并将此值放在多个 BETWEEN 中来创建类别> 条款。这就是我得到的：

    SELECT product_name, unit_price, units_in_stock,
        CASE
            WHEN units_in_stock > (SELECT AVG(units_in_stock) + 10 FROM products) THEN 'many'
            WHEN units_in_stock BETWEEN (SELECT AVG(units_in_stock) - 10 FROM products) AND (SELECT AVG(units_in_stock) + 10 FROM products) THEN 'average'
            ELSE 'low'
        END AS amount
    FROM products
    ORDER BY units_in_stock;

根据 pgAdmin 中的分析工具 AVG(units_in_stock) 计算了 3 次。有没有办法减少计算量？

Answer 1

您可以在子查询中使用窗口函数。另外，在第二个WHEN 条件中不需要使用BETWEEN；大于平均值 + 10 的值由第一个分支处理，永远不会到达第二个分支。

我会这样说：

SELECT product_name, unit_price, units_in_stock,
    CASE
        WHEN units_in_stock >  AVG(units_in_stock) OVER() + 10 THEN 'many'
        WHEN units_in_stock >= AVG(units_in_stock) OVER() - 10 THEN 'average'
        ELSE 'low'
    END AS amount
FROM products
ORDER BY units_in_stock;

我希望数据库能够优化查询，以便只计算一次窗口平均值。如果不是这种情况，另一种方法是先计算子查询中的平均值：

SELECT product_name, unit_price, units_in_stock,
    CASE
        WHEN units_in_stock >  avg_units_in_stock + 10 THEN 'many'
        WHEN units_in_stock >= avg_units_in_stock - 10 THEN 'average'
        ELSE 'low'
    END AS amount
FROM (SELECT p.*, AVG(units_in_stock) OVER() avg_units_in_stock FROM products p) p
ORDER BY units_in_stock;