MySQL 5.7 - Unexpected Behavior with DISTINCT and COUNT in Subqueries
I'm deploying to production and After trying multiple solutions online, I still can't figure this out. Quick question that's been bugging me - I'm running into an issue with subqueries in MySQL 5.7 where I'm trying to count distinct values from a nested query... My goal is to get the number of unique product categories for each user based on their orders. However, my query seems to return incorrect results. Here's the SQL I'm using: ```sql SELECT user_id, COUNT(DISTINCT category_id) AS unique_categories FROM ( SELECT o.user_id, p.category_id FROM orders o JOIN products p ON o.product_id = p.id WHERE o.order_date >= '2023-01-01' ) AS user_products GROUP BY user_id; ``` When I run this query, I expect to see the unique count of categories per user, but the results are unexpectedly high. For example, one user is showing 10 unique categories, even though they should only have purchased from 3. I've checked the data multiple times and verified that the joins are accurate. I've also tried breaking down the query into individual parts to see where the counts might be skewed. Running the inner query alone produces the expected rows, but when I wrap it in the outer query, the counts seem inflated. Additionally, I've confirmed there are no duplicate orders for the same product per user. I even tried adding a `DISTINCT` clause to the inner query: ```sql SELECT DISTINCT o.user_id, p.category_id FROM orders o JOIN products p ON o.product_id = p.id WHERE o.order_date >= '2023-01-01' ``` Unfortunately, that didn’t resolve the issue either. Can anyone provide insight into why my distinct counts are not aligning with my expectations in this subquery setup? Is there a specific behavior in MySQL 5.7 that I might be overlooking? Any help would be greatly appreciated! What's the best practice here? Has anyone else encountered this?