为什么 SUM() 和 COUNT() 返回不同的值？

Question

我有两个几乎相同的查询，我试图了解它们返回不同结果的原因。我想生成一个 table 和一个 user_id 和一个 food_orders 列，显示每个用户订购了多少商品。这些查询产生此 table 但在某些行的 food_orders 列中计算出不同的结果。

我的问题是为什么会这样，我应该使用哪个？

下面是我的查询的简化版本。

查询 1（使用 COUNT）：

WITH order_made AS (
    SELECT restaurant_id,
           count(
                   CASE
                       WHEN item LIKE '%_angus' then 1
                       WHEN item LIKE '%_organic' then 1
                       WHEN item LIKE '%_lean' then 1
                       ELSE 0
                       END)
               AS burgers
    FROM mcdonalds.specialist_orders
    GROUP BY mcdonalds.specialist_orders.user_id
    UNION ALL
    SELECT restaurant_id,
           COUNT(
                   CASE
                       WHEN item LIKE 'salad%' THEN 1
                       WHEN item LIKE 'tomato%' THEN 1
                       WHEN item LIKE 'potatoes%' THEN 1
                       ELSE 0
                       END)
               AS vegetables
    FROM public.bulk_orders
    GROUP BY public.bulk_orders.user_id),

查询 2（使用 SUM）：

WITH orders_made AS (
    SELECT user_id, SUM(food_orders) AS food_orders
    FROM (SELECT user_id,
                   CASE
                       WHEN item LIKE '%_angus' then 1
                       WHEN item LIKE '%_organic' then 1
                       WHEN item LIKE '%_lean' then 1
                       ELSE 0
                       END
               AS food_orders
    FROM mcdonalds.specialist_orders
    UNION ALL
    SELECT user_id,
                   CASE
                       WHEN item LIKE 'salad%' THEN 1
                       WHEN item LIKE 'tomato%' THEN 1
                       WHEN item LIKE 'potatoes%' THEN 1
                       ELSE 0
                       END
               AS food_orders
    FROM public.bulk_orders
    GROUP BY user_id)

Answer 1

因为COUNT函数在值不为NULL（包括0）时才计数，如果不想计数，需要让CASE WHENreturnNULL

因此，如果您从 CASE WHEN 中删除 ELSE 0 或使用 ELSE NULL 代替，结果将与 SUM

相同

WITH order_made AS (
    SELECT restaurant_id,
           count(
                   CASE
                       WHEN item LIKE '%_angus' then 1
                       WHEN item LIKE '%_organic' then 1
                       WHEN item LIKE '%_lean' then 1
                       END)
               AS burgers
    FROM mcdonalds.specialist_orders
    GROUP BY mcdonalds.specialist_orders.user_id
    UNION ALL
    SELECT restaurant_id,
           COUNT(
                   CASE
                       WHEN item LIKE 'salad%' THEN 1
                       WHEN item LIKE 'tomato%' THEN 1
                       WHEN item LIKE 'potatoes%' THEN 1
                       END)
               AS vegetables
    FROM public.bulk_orders
    GROUP BY public.bulk_orders.user_id),

为什么 SUM() 和 COUNT() 返回不同的值？

Why is SUM() and COUNT() returning different values?

sql

group-by

count