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

Question

I have two near identical queries and I'm trying to understand why they're returning different results. I would like to produce a table with a user_id and a food_orders column showing how many items each user has ordered. These queries produce this table but calculate different results in the food_orders column in some rows.
My questions are why is this and which should I use?
A simplified version of my queries are below.
Query 1 (using 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),
Query 2 (using 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

Because COUNT function count when the value isn't NULL (include 0) if you don't want to count, need to let CASE WHEN return NULL
so if you remove ELSE 0 from your CASE WHEN or use ELSE NULL to instead, that result would be as same as SUM
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),