MySQL: How to efficiently update rows based on a calculated average from a subquery?
I'm trying to update a column in my MySQL database based on the average value of another column from a related table, but I'm running into performance issues and unexpected results. My setup includes a `users` table and a `transactions` table where each user can have multiple transactions. I'm trying to update the `users` table's `average_transaction_value` column with the average of the `amount` from the `transactions` table for each user. Here’s the SQL query I’m currently using: ```sql UPDATE users u SET average_transaction_value = ( SELECT AVG(t.amount) FROM transactions t WHERE t.user_id = u.id ) WHERE EXISTS ( SELECT 1 FROM transactions t WHERE t.user_id = u.id ); ``` Although the query seems correct at first glance, it's taking a long time to execute, especially since I have over a million records in the `transactions` table. Additionally, I noticed that some `average_transaction_value` entries are `NULL` after the update, despite users having transactions. I’ve created an index on `transactions.user_id` to speed up the subquery, but it didn’t significantly affect performance. I’ve also ensured that my MySQL server is using version 8.0.26, which should support the optimizations I’m attempting. I’ve tried breaking the query into a temporary table to store averages first and then updating the `users` table, but that approach was also slow and complicated. What can I do to improve the efficiency of this update, and how can I ensure that all users with transactions get their `average_transaction_value` updated correctly? Any suggestions would be greatly appreciated! What's the best practice here?