通过外键查找Max（）值

Question

Consider this database structure:

 __________                 __________
| Trucks   |               | Mileage  |
|__________|__________     |__________|________________________
| ID    | DRIVER      |    | TRUCK_ID  | MILEAGE  | OIL_CHANGE |
|---------------------|    |-----------------------------------|
| 1     | Tony        |    | 1         | 100000     105000     |
| 2     | George      |    | 2         | 6020       10020      |
| 3     | Mary        |    | 3         | 37798      41000      |
|_____________________|    | 3         | 41233      47200      |
                           | 3         | 49000                 |
                           |___________________________________|

I want to end up with a result set containing the maximum miles and maximum oil_change for each driver.

_________________________________
| 1  | Tony  |  100000 | 105000 |
| 2  | George|  6020   | 10020  |
| 3  | Mary  |  49000  | 47200  |
|_______________________________|

This is what I have tried so far:

SELECT t.*, MAX(m.mileage) AS mileage, MAX(m.oil_change) AS oil_change
FROM trucks t
LEFT JOIN mileage m ON t.id = m.truck_id
GROUP BY t.id

But this doesn't seem to allow the MAX function to work properly. It does not always contain the actual maximum value for mileage

Answer 1

Got it! Your mileage column must be defined as a character type, not a numeric type! When than happens, order is done alphabetically, not by value.

You should convert your mileage and oil_change columns to a numeric type (I'd recommend INT based on the data sample provided).

While you don't convert them, this will work:

SELECT t.*, MAX(cast(m.mileage as int)) AS mileage, 
            MAX(cast(m.oil_change as int)) AS oil_change
FROM trucks t
LEFT JOIN mileage m ON t.id = m.truck_id
GROUP BY t.id

Answer 2

The below queries should work for your question.

SELECT T.DRIVER,MIN(MILEAGE) AS MIN_MILEAGE,MIN(OIL_CHANGE) AS MIN_OIL_CHANGE
FROM TRUCKS T INNER JOIN MILEAGE M
ON T.ID = M.TRUCK_ID
GROUP BY T.DRIVER;


SELECT T.DRIVER,MAX(MILEAGE) AS MAX_MILEAGE,MAX(OIL_CHANGE) AS MAX_OIL_CHANGE
FROM TRUCKS T INNER JOIN MILEAGE M
ON T.ID = M.TRUCK_ID
GROUP BY T.DRIVER;

Regards Venk