[Solved] MySQL - Selecting a Column not in Group By

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MySQL - Selecting a Column not in Group By

Database type:

I'm trying to add features to a preexisting application and I came across a MySQL view something like this:

SELECT
     AVG(table_name.col1),
     AVG(table_name.col2),
     AVG(table_name.col3),
     table_name.personID,
     table_name.col4
FROM table_name
GROUP BY table_name.personID;

OK so there's a few aggregate functions. You can select personID because you're grouping by it. But it also is selecting a column that is not in an aggregate function and is not a part of the GROUP BY clause. How is this possible??? Does it just pick a random value because the values definitely aren't unique per group?

Where I come from (MSSQL Server), that's an error. Can someone explain this behavior to me and why it's allowed in MySQL?

How to optimize this SQL query?

The following recommendations will help you in your SQL tuning process.
You'll find 3 sections below:

  1. Description of the steps you can take to speed up the query.
  2. The optimal indexes for this query, which you can copy and create in your database.
  3. An automatically re-written query you can copy and execute in your database.
The optimization process and recommendations:
  1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
  2. Explicitly ORDER BY After GROUP BY (modified query below): By default, the database sorts all 'GROUP BY col1, col2, ...' queries as if you specified 'ORDER BY col1, col2, ...' in the query as well. If a query includes a GROUP BY clause but you want to avoid the overhead of sorting the result, you can suppress sorting by specifying 'ORDER BY NULL'.
Optimal indexes for this query:
ALTER TABLE `table_name` ADD INDEX `table_name_idx_personid` (`personID`);
The optimized query:
SELECT
        AVG(table_name.col1),
        AVG(table_name.col2),
        AVG(table_name.col3),
        table_name.personID,
        table_name.col4 
    FROM
        table_name 
    GROUP BY
        table_name.personID 
    ORDER BY
        NULL

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* original question posted on StackOverflow here.