Order by and group by - mysql

Question

I am trying to show delivery charges for a shop I am building, there are three tables in the database 1 for the service ie Royal Mail, Carrier..., one for the band ie. UK, Europe, Worldwide1 etc.. and one for the charges (qty = weight)
I have a database of three tables that, when joined form the following
+------------------+-----+-----------+-------+---------+---------------+----------+-------+-------------+
| name | qty | serviceID | basis | bandID | initial_charge | chargeID | price | total_price |
+------------------+-----+-----------+-------+---------+---------------+----------+-------+-------------+
| Collect in store | 0 | 3 | | 1 | 3 | 0.00 | 2 | 0.00 |
| Royal mail | 0 | 1 | 2 | 4 | 2.00 | 3 | 0.00 | 2.00 |
| Royal mail | 1 | 1 | 2 | 4 | 2.00 | 4 | 1.00 | 3.00 |
| APC | 0 | 2 | 1 | 1 | 0.00 | 6 | 5.95 | 5.95 |
+------------------+-----+-----------+-------+---------+---------------+----------+-------+-------------+
Basically what I want to do is (as you can see) Royal Mail has two entries as there are more than one entry in the joined table. What I would like to do is show the highest of the two royal mail entries (I was initially trying to group by service_id) whilst also maintaining the two other services with different service id's
Any assistance would be great as this is driving me mad. I feel like I have tried every combination going!
In the example below the qty (weight) of the items is 3kg
SELECT
`service`.`name`,
`charge`.`qty`,
`service`.`serviceID`,
`band`.`bandID`,
`band`.`initial_charge`,
`charge`.`chargeID`,
`charge`.`price`,
`band`.`initial_charge` + `charge`.`price` AS `total_price`
FROM
`delivery_band` AS `band`
LEFT JOIN
`delivery_charge` AS `charge`
ON
`charge`.`bandID` = `band`.`bandID`
AND
`charge`.`qty` < '3'
LEFT JOIN
`delivery_service` AS `service`
ON
`service`.`serviceID` = `band`.`serviceID`
WHERE
FIND_IN_SET( '225', `band`.`accepted_countries` )
AND
(
`band`.`min_qty` >= '3'
OR
`band`.`min_qty` = '0'
)
AND
(
`band`.`max_qty` <= '3'
OR
`band`.`max_qty` = '0'
)
delivery_service
+-----------+------------------+
| serviceID | name |
+-----------+------------------+
| 1 | Royal mail |
| 2 | APC |
| 3 | Collect in store |
+-----------+------------------+
delivery_band
+--------+-----------+-----------------+----------------+---------+---------+-------------------------------------------------------+
| bandID | serviceID | name | initial_charge | min_qty | max_qty | accepted_countries |
+--------+-----------+-----------------+----------------+---------+---------+-------------------------------------------------------+
| 1 | 2 | UK Mainland | 0.00 | 0 | 0 | 225 |
| 2 | 2 | UK Offshore | 14.00 | 0 | 0 | 240 |
| 3 | 3 | Bradford Store | 0.00 | 0 | 0 | 225 |
| 4 | 1 | UK | 2.00 | 0 | 0 | 225 |
| 5 | 2 | World wide | 15.00 | 0 | 0 | 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20... |
| 6 | 1 | World wide Mail | 5.00 | 0 | 0 | 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20... |
+--------+-----------+-----------------+----------------+---------+---------+-------------------------------------------------------+
delivery_charge
+----------+--------+-----+-------+
| chargeID | bandID | qty | price |
+----------+--------+-----+-------+
| 1 | 2 | 0 | 5.00 |
| 2 | 3 | 0 | 0.00 |
| 3 | 4 | 0 | 0.00 |
| 4 | 4 | 1 | 1.00 |
| 5 | 4 | 5 | 3.00 |
| 6 | 1 | 0 | 5.95 |
| 7 | 1 | 10 | 10.95 |
| 8 | 2 | 10 | 14.00 |
| 9 | 5 | 0 | 0.00 |
| 10 | 5 | 3 | 5.00 |
| 11 | 5 | 6 | 10.00 |
| 12 | 5 | 9 | 15.00 |
| 13 | 6 | 0 | 0.00 |
| 14 | 6 | 2 | 5.00 |
| 15 | 6 | 4 | 10.00 |
| 16 | 6 | 6 | 15.00 |
+----------+--------+-----+-------+
When I tried adding the charge table as a sub query and then limiting that query it gave me NULL's for all the charge table fields
If I try the following query:
SELECT
`service`.`name`,
`charge`.`qty`,
`service`.`serviceID`,
`band`.`bandID`,
`band`.`initial_charge`,
`charge`.`chargeID`,
MAX( `charge`.`price` ) AS `price`,
`band`.`initial_charge` + `charge`.`price` AS `total_price`
FROM
`delivery_band` AS `band`
LEFT JOIN
`delivery_charge` AS `charge`
ON
`charge`.`bandID` = `band`.`bandID`
AND
`charge`.`qty` < '3'
LEFT JOIN
`delivery_service` AS `service`
ON
`service`.`serviceID` = `band`.`serviceID`
WHERE
FIND_IN_SET( '225', `band`.`accepted_countries` )
AND
(
`band`.`min_qty` >= '3'
OR
`band`.`min_qty` = '0'
)
AND
(
`band`.`max_qty` <= '3'
OR
`band`.`max_qty` = '0'
)
GROUP BY
`service`.`serviceID`
I get this returned:
+------------------+-----+-----------+--------+----------------+----------+-------+-------------+
| name | qty | serviceID | bandID | initial_charge | chargeID | price | total_price |
+------------------+-----+-----------+--------+----------------+----------+-------+-------------+
| Royal mail | 0 | 1 | 4 | 2.00 | 3 | 1.00 | 2.00 |
| APC | 0 | 2 | 1 | 0.00 | 6 | 5.95 | 5.95 |
| Collect in store | 0 | 3 | 3 | 0.00 | 2 | 0.00 | 0.00 |
+------------------+-----+-----------+--------+----------------+----------+-------+-------------+
Which looks fine in principle until you realise that the chargeID = 3 has a price of 0.00 and yet the table is showing a price of 1.00 so the values seem to have become disassociated

Answer 1

What I would like to do is show the highest of the two royal mail entries
You can use MAX to obtain the maximum of a given column, e.g.
SELECT … MAX(charge.price) … FROM …
If you absolutely need the other columns (like charge.chargeID) to match, things will become a lot more complicated. So make sure you actually need that. For details on the general idea behind this kind of query, have a closer look at Select one value from a group based on order from other columns. Adapting this answer by #RichardTheKiwi, I came up with the following query:
SELECT s.name,
c.qty,
s.serviceID,
b.bandID,
b.initial_charge,
c.chargeID,
c.price,
b.initial_charge + c.price AS total_price
FROM delivery_band AS b,
delivery_service AS s,
(SELECT chargeID, price, qty,
#rowctr := IF(bandId = #lastBand, #rowctr+1, 1) AS rowNumber,
#lastBand := bandId AS bandId
FROM (SELECT #rowctr:=0, #lastBand:=null) init,
delivery_charge
WHERE qty < 3
ORDER BY bandId, price DESC
) AS c
WHERE FIND_IN_SET(225, b.accepted_countries)
AND (b.min_qty >= 3 OR B.min_qty = 0)
AND (b.max_qty <= 3 OR B.max_qty = 0)
AND s.serviceID = b.serviceID
AND c.bandID = b.bandID
AND c.rowNumber = 1
See this fiddle for the corresponding output. Note that I only do inner queries, not left queries, since that seems sufficient for the query in question, and keeps things a lot more readable so you can concentrate on the important parts, i.e. those involving rowNumber. The idea is that the subquery generates row numbers for the items of the same band, resetting them for the next band. When you select only rows with rowNumber being 1, you only get the highest price, with all other columns associated with that.