How to Retrieve the Most Recent Email from Each Sender Using MySQL's GROUP BY and ORDER BY?

Detailed explanation and application of MySQL's GROUP BY and ORDER BY clauses

When working with data tables in a MySQL database, it is crucial to understand the interaction of the GROUP BY and ORDER BY clauses. Consider the following scenario:

You need to group the rows in the email table by the sender's email address, with the goal of retrieving the latest email from each sender. However, the following query:

SELECT 
    `timestamp`, `fromEmail`, `subject`
FROM `incomingEmails` 
GROUP BY LOWER(`fromEmail`) 
ORDER BY `timestamp` DESC

will produce incorrect results. It groups emails by sender, but may not show the latest topics in each group. Results may show:

<code>fromEmail: [email protected], subject: hello
fromEmail: [email protected], subject: welcome</code>

And the actual data contains:

<code>fromEmail: [email protected], subject: hello
fromEmail: [email protected], subject: programming question
fromEmail: [email protected], subject: welcome</code>

Solution: Use subquery and ANY_VALUE() function

To solve this problem, you can use a subquery in combination with the ANY_VALUE() function:

SELECT * 
FROM ( 
    SELECT `timestamp`, `fromEmail`, ANY_VALUE(`subject`) AS `subject`
    FROM `incomingEmails` 
    ORDER BY `timestamp` DESC
) AS tmp_table 
GROUP BY LOWER(`fromEmail`)

In MySQL 5.7 and later, ANY_VALUE() allows the use of non-aggregated columns in a GROUP BY statement with a SELECT clause. It randomly selects a value from each group, ensuring that the subquery retrieves the most recent subject for each sender's email address.

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