SQL GROUP BY multiple columns with one non-repeating column - sql

Question

I am attempting to query multiple columns in order to display the heaviest ship for each builder/company name.
When using my above query I instead receive the results for every ships weight instead of the heaviest ship for each builder. I have spent a few hours trying to discern what is needed to cause the builder column to be distinct.

Answer 1

You have to remove 'shipname' from the group by list to get the max weight for each builder, then join the query to the original table to get the ship name as the following:
select T.builder, T.shipname, T.weight
from ship T join
(
select builder, max(weight) mx
from ship
group by builder
) D
on T.builder=D.builder and T.weight=D.mx
order by T.builder
You may also use DESNE_RANK() function to get the required results as the following:
select top 1 with ties
builder, shipname, weight
from ship
order by dense_rank() over (partition by builder order by weight desc)
See a demo on SQL Server (I supposed that you are using SQL Server from the posted image).

Answer 2

You don't need to apply a GROUP BY clause in this situation. It will be sufficient to check whether the ship's weight is the highest weight for the current builder. This can be done with a simple sub query:
SELECT
builder, shipname, weight
FROM
ship
WHERE
weight = (SELECT MAX(i.weight) FROM ship i WHERE i.builder = ship.builder)
ORDER BY builder;

Answer 3

I always think the simplest way to handle a complex query starts with a Common Table Expression (CTE). If you’re new to this, a CTE is a query which is run as a first step, so that you can use its results in the next step.
WITH cte AS (
SELECT builder, max(weight) AS weight
FROM data
GROUP BY builder
)
SELECT *
FROM cte JOIN data ON cte.builder=data.builder AND cte.weight=data.weight;
The CTE above fetches the rows with the maximum weights:
SELECT builder, max(weight) AS weight
FROM data
GROUP BY builder
builder
weight
Ace Shipbuilding Corp
95000
Ajax
90000
Jones
95000
Master
80000
You now have the maximum weight for each builder.
You then join this result with the original data to fetch the rows which match the weights and builders:
builder
weight
builder
shipname
weight
Master
80000
Master
Queen Shiney
80000
Jones
95000
Jones
Princess of Florida
95000
Ajax
90000
Ajax
Prince Al
90000
Ace Shipbuilding Corp
95000
Ace Shipbuilding Corp
Ocean V
95000
Ace Shipbuilding Corp
95000
Ace Shipbuilding Corp
Sea Peace
95000
Note that there is a tie for the Ace Shipbuilding Corp.
The above solution supposes that the data you have in your sample is the original data.