How to SELECT only one record from a group of related records in SQL?

Question

I want to GROUP rows together based on a field, then SELECT only 1 of those fields from each group.

Say I have a ProductID which is a compound key key based on 4 fields {model, make, region, iteration}. The ProductID is always in the format 00-0000-00000-00, and the iteration is a sequentially incremented integer. I can group all of the products together where the model, make and region are the same using the below SQL:

SELECT pt1.ProductID
FROM ProductsTable pt1
    INNER JOIN ProductsTable2 pt2 ON pt1.ProductID = pt2.ProductID
GROUP BY LEFT(pt1.ProductID, 13)

which would have one GROUP something like:

3A-CSTC,00246-01
3A-CSTC,00246-02
3A-CSTC,00246-03

How would I return only the row that has the highest iteration in the ProductID (in this case 3A-CSTC,00246-03)?

Data Definitions

TABLE_NAME     COLUMN_NAME  PRIMARY_KEY   DATA_TYPE   IS_NULLABLE
Models         ModelID      TRUE          char(2)     NO
Makes          MakeID       TRUE          char(4)     NO
Regions        RegionID     TRUE          char(5)     NO
Iterations     IterationId  TRUE          char(2)     NO
ProductTable1  ProductID    TRUE          char(16)    NO
ProductTable2  ProductID    TRUE          char(16)    NO

So the ProductId = ModelID + "-" + MakeID + "-" + RegionID + "IterationId"

I am aware of the HAVING clause; however, I have no idea on what expression I could use. I did come across a similar, but different question.

Answer 1

I don't fully comprehend that data scenario your dealing with, when you say you want to return one row, from what table do you want the row from, and what table is causing the multiple records? selecting a simple row from the db is not possible when you are grouping. You would have to aggregate everything you are trying to select. It sounds like you need to leverage your primary key on one of the tables and do a sub-select of some type.

select * from ProductsTable pt1
join (select left(pt1.ProductID, 13) as prodid, max(pt2.primarykey) as primarykey from ProductsTable pt2 group by left(pt1.ProductID, 13)) as a on a.primarykey=pt1.primarykey

Answer 2

You should probably GROUP BY a substring of the ProductID and SELECT using an aggregate function:

SELECT MAX(pt1.ProductID)
FROM ProductsTable pt1
    INNER JOIN ProductsTable2 pt2 ON pt1.ProductID = pt2.ProductID
GROUP BY LEFT(pt1.ProductID,13)

This is assuming that the ProductID remains a consistent length and you can count on the iterations not going over two digits.