SQL Server - how to to Optimize Query with Multiple INNER JOINs and DISTINCT
I'm attempting to set up I've looked through the documentation and I'm still confused about I'm trying to figure out I'm stuck on something that should probably be simple. I'm working on a SQL Server 2019 query that involves multiple INNER JOINs across three tables, but I'm working with performance optimization and unexpected results. My goal is to retrieve distinct records based on a combination of columns from these tables, but the query takes a long time to execute and sometimes returns duplicates. Here’s the basic structure of my query: ```sql SELECT DISTINCT a.id, b.name, c.amount FROM TableA a INNER JOIN TableB b ON a.id = b.a_id INNER JOIN TableC c ON b.id = c.b_id WHERE c.amount > 100; ``` I've tried adding indexes on the columns used for joining and filtering, but it hasn't helped much. The execution plan shows a lot of key lookups, which I suspect is causing the slowdown. I also considered using CTEs or subqueries, but that didn’t yield better performance either. Additionally, when I run the query, I sometimes get duplicate results, which I thought using DISTINCT would eliminate. Here’s the query plan showing the joins: ```plaintext |--Nested Loops(Inner Join, OUTER:([b].[id]=[c].[b_id])) |--Nested Loops(Inner Join, OUTER:([a].[id]=[b].[a_id])) |--Index Scan(TableA) |--Index Scan(TableB) |--Index Scan(TableC) ``` I've also tried filtering on the joins themselves, like adding more conditions in the ON clause, but it seems to worsen the performance. Could there be a better way to structure this query or optimize it further to avoid duplicates and improve execution time? Any insights into why the DISTINCT is not functioning as expected would also be greatly appreciated! Any help would be greatly appreciated! My development environment is Windows 10. Is there a better approach? I'm using Sql 3.11 in this project. Is there a simpler solution I'm overlooking? I've been using Sql for about a year now. Thanks for your help in advance! What's the correct way to implement this?