MySQL: implementing UNION ALL causing duplicate rows despite different SELECT statements
I'm working on a project and hit a roadblock. After trying multiple solutions online, I still can't figure this out. I'm working with unexpected behavior when using `UNION ALL` in MySQL 8.0... I have two separate queries that should yield distinct results, but after combining them, I see duplicated rows that I don't expect. Here's the code I'm working with: ```sql SELECT id, name, 'source1' AS source FROM table1 WHERE active = 1 UNION ALL SELECT id, name, 'source2' AS source FROM table2 WHERE active = 1; ``` Both `table1` and `table2` have unique `id` values, but when I run the combined query, the output sometimes shows the same `id` from both tables, which leads me to believe there might be an scenario in the way I'm structuring the query or the data itself. I've verified that both tables have unique values prior to applying the `UNION ALL`, and I even added a temporary filter in the `WHERE` clause to double-check the uniqueness: ```sql SELECT id, COUNT(*) FROM ( SELECT id FROM table1 WHERE active = 1 UNION ALL SELECT id FROM table2 WHERE active = 1 ) AS combined GROUP BY id HAVING COUNT(*) > 1; ``` This query returns an empty result set, which is what I expect. However, when I run the original `UNION ALL`, I still see duplicates in the output. Is there something I might be overlooking regarding how MySQL processes these queries? Could it be related to how the two tables are joined logically, or should I be using `UNION` instead of `UNION ALL`? Any insights would be greatly appreciated! My development environment is Windows. Any help would be greatly appreciated! My development environment is Windows. How would you solve this? I'm working in a Windows 10 environment. Has anyone dealt with something similar?