Question

I have two queries, the only difference being the GROUP BY clause

SELECT * FROM `packages_sorted_YHZ` WHERE `hotel_city` = 'Montego Bay'
ORDER BY  `deal_score` DESC 
LIMIT 0,3;

SELECT * FROM `packages_sorted_YHZ` WHERE `hotel_city` = 'Montego Bay'
GROUP BY `hotel_name`
ORDER BY  `deal_score` DESC
LIMIT 0,3;

The first query returns the first result with a deal_score of 75 but the second query returns the first result with the deal_score of just 72.

I would have thought that regardless of the GROUP BY clause, the first result would have the highest deal score possible (75)

The purpose of the GROUP BY clause is to optionally select a unique hotel_name for each result.
Does anyone know what I’m doing wrong here.

Answer 1

Without being able to look at all the data, my best guess is that Group By is merging the data and giving you an arbitrary value that matches the Where clause. This will happen if hotel name isn’t unique, and you won’t be given the maximum score unless you specifically query for it.

Try putting a Max() around deal_score. In MySQL, Group By can be used way too easily, I like how MSSQL enforces the use of aggregate functions and grouping by every field that isn’t aggregated. Try this query:

SELECT `hotel_name`, MAX( `deal_score` ) AS `max_score` FROM `packages_sorted_YHZ` WHERE `hotel_city` = 'Montego Bay'
GROUP BY `hotel_name`
ORDER BY  `max_score` DESC
LIMIT 0,3;