Question

The following SQL query:

SELECT messages.id, messages.created_at, comments.created_at FROM messages
LEFT JOIN comments ON comments.message_id = messages.id 
WHERE (messages.id IN (429,443)) 
ORDER BY GREATEST(messages.created_at, comments.created_at) DESC

returns:

 id         messages.created_at     comments.created_at
--------------------------------------------------------
 443                2                       5
 429                1                       4
 443                2                       3

 (I replaced dates with numbers for readability)

To get each id only once I added DISTINCT:

SELECT DISTINCT messages.id FROM messages
LEFT JOIN comments ON comments.message_id = messages.id 
WHERE (messages.id IN (429,443)) 
ORDER BY GREATEST(messages.created_at, comments.created_at) DESC

But, in the result the id values changed order:

id
---
429
443

What could be the reason for that ?

How could I keep the order ?

Answer 1

the distinct key word is doing what it’s supposed to do, return one row each with a given column value. Distinct doesn’t allow you to specify which such row will be returned, and it’s clear from the original query that such an ordering is allowed (there is a row with id 443 that follows a row with id 429).

To take control of what rows will be returned, you need to reformulate the query. A typical solution I’ll take is to use a group by, selecting the group column and the desired row from each group, something to the effect of

SELECT message.id, MAX(message.created_at) FROM message GROUP BY message.id;

If I need to do more, I’ll use this sort of query as a subselect in a larger query, possibly joining on the id field to get more fields from the preferred row, Or ordering the query in a particular way.