CODEWITHSUNDEEP
sqlgreatest-n-per-group
JD Gamboa
JD Gamboa

Reputation: 382

Select distinct values from a column based on highest value in another column

I have a table in which I need to return a distinct value for each id based on the highest reference value. As an example, this table:

my table
|--------|----------|-------------|
| ID     | Amount   | Ref_value   |
|---------------------------------|
| 1      | 200      | 5           |
| 1      | 120      | 8           |
| 2      | 150      | 3           |
| 3      | 200      | 4           |
|--------|----------|-------------|

I need to get one return per ID, but since ID=1 appears more than once, I need to select the one with the highest "Ref_value". So my result would be:

result of the query over my table
|--------|----------|-------------|
| ID     | Amount   | Ref_value   |
|---------------------------------|
| 1      | 120      | 8           |
| 2      | 150      | 3           |
| 3      | 200      | 4           |
|--------|----------|-------------|

Upvotes: 2

Views: 6522

Answers (3)

Vash
Vash

Reputation: 1787

Select * might throw an ambiguous column names error. So you might want to select the only required columns there. 

SELECT A.ID, A.AMOUNT, A.REF_VALUE
FROM
MYTABLE A
INNER JOIN
(SELECT ID, MAX(REF_VALUE) AS MAX_REF FROM MYTABLE GROUP BY ID) B
ON A.ID = B.ID AND A.REF_VALUE = B.MAX_REF;

Upvotes: 1

Lukas Eder
Lukas Eder

Reputation: 220762

If your database supports window functions, this solution will access the table only once:

SELECT id, amount, ref_value
FROM (
  SELECT t.*, row_number() OVER (PARTITION BY id ORDER BY ref_value DESC) rn
  FROM t
) t
WHERE rn = 1

If you're using Oracle, you might use this solution that I've found to be a bit faster in most cases:

SELECT
  max(id)        KEEP (DENSE_RANK FIRST ORDER BY ref_value DESC) id,
  max(amount)    KEEP (DENSE_RANK FIRST ORDER BY ref_value DESC) amount,
  max(ref_value) KEEP (DENSE_RANK FIRST ORDER BY ref_value DESC) ref_value,
FROM t
GROUP BY id

Both of the above solutions will return an arbitrary row if two ref_value values are tied.

If none of the above apply, scaisEdge's solution works on all databases.

Upvotes: 2

ScaisEdge
ScaisEdge

Reputation: 133360

you could use a inner join on the max value grouped by id

select * from my_table 
inner join (
   select id, max(ref_value) max_value
   from my_table 
   group by id
)  t on t.id = my_table_id, t.max_vale = my_table_value

Upvotes: 3

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