Question

I’ve mixed up with a strange behavior of MySQL query. I have next mysql query:

SELECT 'username','status', 'field_1', 'field_2', 'field_3', 'field_4',  
    FROM my_table 
    ORDER by field_1 DESC, field_2 DESC, field_3 DESC, field_4 DESC 
    LIMIT 0,10 

By the idea, it has to order 10 rows in descending method depending on how many values fields have in ORDER BY condition. But in the result I get the next one:

kate 103
pete 101
steve 102

instead of

kate 103
steve 102
pete 101

Does anyone know why it set incorrect order? And what to do in order to make the proper ORDER BY DESC condition?

Is it possible to use MAX() for several fields? If yes, maybe it is possible to organize the MySQL query like this?

SELECT 'username','status', 'field_1', 'field_2', 'field_3', 'field_4', MAX(field_1,field_2,field_3,field_4) AS total 
    FROM my_table 
    ORDER by total DESC 
    LIMIT 0,10

Answer 1

You can’t use MAX() because that function returns the largest value in one column over many rows. But you can use MySQL’s GREATEST() function. This returns the largest of its arguments, which all come from one row.

SELECT `username`, `status`, GREATEST(field_1,field_2,field_3,field_4) AS field_greatest
FROM my_table
ORDER BY field_greatest DESC
LIMIT 0,10

But I would also comment that you have violated 1st Normal Form by storing multiple columns that should be the same "kind" of data. The consequences are that you face this troublesome query. Also you’ll need to rewrite all your queries when you add a fifth field.

Instead, create a second table and store all the "fields" in a single column, with a reference to the user. Then you can join the tables:

SELECT t.username, t.status, f.field
FROM my_table AS t
LEFT OUTER JOIN (SELECT username, MAX(field) AS field FROM my_fields GROUP BY username) AS f
  ON t.username = f.username
ORDER BY f.field DESC
LIMIT 0,10

Or my favorite way of getting the row with the greatest value per group:

SELECT t.username, t.status, f1.field
FROM my_table AS t
JOIN my_fields AS f1 ON t.username = f1.username
LEFT OUTER JOIN my_fields AS f2 ON t.username = f2.username AND f1.field < f2.field
WHERE f2.username IS NULL
ORDER BY f1.field DESC
LIMIT 0,10