Question

I have this query working to some extent.
It returns the correct value for ‘rating’ (which output as 7, the highest rating), but the output for ‘content’ is from a different row in the table. (not the row of the highest rating, which is 7)

$bestAnswerQuery = MYSQL_QUERY("SELECT content, MAX(rating) as rating FROM answers WHERE questionID = '$questionID'");

$fetchBestAnswer = MYSQL_FETCH_ASSOC($bestAnswerQuery);

echo "$fetchBestAnswer[content] $fetchBestAnswer[rating]";

Can anyone tell me why? I’ve searched and cannot find out why this isn’t working properly.

Answer 1

This is not how aggregates like MAX work in SQL. Your confusion is coming from MySQL’s (default) non-ANSI handling of aggregates.

Aggregates like MAX operate over groups. In the absence of a group by clause, the entire result set is considered to be a single group. Only expressions that are part of a group by clause can be included in a select clause without being enclosed in an aggregate. In the case where there is no group by, then all columns or expressions in the select clause must be contained in an aggregate.

However, MySQL’s default configuration breaks this by allowing you to include non-grouped expressions in the select clause, but the row that any given expression uses to obtain its value is undefined; it could be any row within the group.

After that long-winded answer, if what you want to get is the maximum rating and the associated content column from the table for a given question, you can just do this:

select 
    rating, 
    content 

from answers 

where questionID = '$questionID' 

order by rating desc 

limit 1;