Mysql "Group By" and "Order By"

Advanced practices and tricks

Much as the previous command works perfectly for a typical count-and-sort operation, more advanced uses of GROUP BY and ORDER BY require a deeper understanding. Let's explore some advanced scenarios.

Retrieving recent data for each group

To fetch the most recent entry for each email, you might want to first ORDER BY the timestamp and then GROUP BY:

-- "The early bird gets the worm... But the second mouse gets the cheese" 😄
SELECT main.*
FROM table AS main
INNER JOIN (
    SELECT email, MAX(timestamp) AS max_timestamp
    FROM table
    GROUP BY email
) AS sub ON main.email = sub.email AND main.timestamp = sub.max_timestamp;


Avoiding unpredictability with ANY_VALUE()

A GROUP BY clause using non-aggregate columns without ANY_VALUE() can return non-deterministic results:

-- Don't play Russian Roulette with your data! 😉
SELECT column, any_non_aggregate_column, MAX(some_column) 
FROM table
GROUP BY column;


To avoid unpredictable output, exploit ANY_VALUE() or make sure all non-aggregate columns in SELECT are part of GROUP BY.

Handling ONLY_FULL_GROUP_BY in MySQL

Understanding the ONLY_FULL_GROUP_BY mode is paramount. When enabled, MySQL will reject your queries that defy SQL standards:

-- Be a law-abiding SQLizen! 💂
SELECT column, ANY_VALUE(other_column) 
FROM table 
GROUP BY column;


Importance of data indexing

Indexing is one of the key players in query performance. Make sure your indexes align with the columns used in JOIN, WHERE, GROUP BY, and ORDER BY:

-- I don't have time... because my index went for a coffee break! ☕
SELECT * FROM table WHERE indexed_column = 'value';


Mastering subqueries for control over sorting

When you need to fetch the most recent record per group, a subquery with ORDER BY can help sort out data prior to the final GROUP BY operation:

-- Like Marie Kondo, I love when things are nicely sorted! 🧹🧽
SELECT a.*
FROM (SELECT * FROM table ORDER BY timestamp DESC) a
GROUP BY a.email;


Embrace the philosophy of simplicity

Remember to keep your SQL statements simple and clean. Avoid unnecessary complexity in your queries - unless it adds robust values.

Time-paradox? Use a trusty ID

When records could have identical timestamps, an incremental ID can act as your savior.

-- "Time keeps on slippin'... into the same exact second?" 🕰️
SELECT email, MAX(timestamp), MAX(id) AS max_id
FROM table
GROUP BY email;


You can join this result back to the table to retrieve the remaining columns.

References