How to Select a Random Subset and Sort by a Specific Field in MySQL?

Ensuring Randomness with MySQL ORDER BY

When handling a large dataset, it's often necessary to extract a random subset of records and then further sort them based on specific criteria. MySQL's ORDER BY clause provides the functionality to achieve this.

Let's consider a scenario where we have a database table users with 1000 records. To select 20 random users and order the results by their names in ascending order, we might initially use this query:

SELECT * FROM users WHERE 1 ORDER BY rand(), name ASC LIMIT 20

However, this query doesn't guarantee a truly randomized result. The ORDER BY rand() part shuffles the records, but the WHERE 1 condition essentially cancels out the randomness by selecting all records.

To achieve the desired behavior, we employ a subquery to isolate the random selection process:

SELECT * FROM
(
    SELECT * FROM users ORDER BY rand() LIMIT 20
) T1
ORDER BY name

The inner query selects 20 random users using ORDER BY rand() and LIMIT 20. The outer query then applies the ORDER BY name clause to arrange the selected users in alphabetical order. This approach produces a random subset of records that are subsequently sorted by the desired field, providing the intended results.

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