Merge values in sql rows

Question

I am looking for ways to merge row values into one row where the column to merge is the same

Transform:

FK | F1 
========
3  | ABC    
3  | DEF 

to

FK | F1    | F2
=================
3  | ABC   | DEF

Update: I initially don`t know the values of F1. They might be everything, but I know they are unique for a given FK and they are varchars.

Update 2: With your help I came to this query that will also add the FK for which there is only one value. I suppose it could be improved.

SELECT IFNULL(jointable.FK, table .FK) AS FK, IFNULL(jointable.F1, table .F1), jointable.F2 FROM table LEFT JOIN (SELECT T1.FK, T1.F1, T2.F1 AS F2 FROM table T1 LEFT JOIN table T2 ON T1.FK = T2.FK WHERE T1.F1 <> T2.F1 GROUP BY T1.FK ) as jointable ON table .FK=jointable.FK GROUP BY FK;

Answer 1

Try this

SELECT FK
     , T1.F1
     , T2.F1 AS F2
FROM table T1
LEFT JOIN table T2 ON T1.FK = T2.FK AND T1.F1 <> T2.F1 --Criteria moved here

The LEFT JOIN is used since you mentioned that you have 1 or more values, which means the INNER JOIN could end up excluding rows.

The second criteria is to make sure you don't en up with rows like:

FK | F1    | F2
=================
3  | ABC   | ABC

Please be aware that in case of an OUTER JOIN (either LEFT or RIGHT ) the join criteria is not the same as the filter criteria, and therefore I moved it above.

In SQL Server, you can use ROW_NUMBER() over FK , maybe with an ORDER BY .

In MySQL you might be able to use it with a GROUP BY as you mentioned in comments, I am not sure it will work (at least not in SQL Server without an aggregate function or a CTE).

Here is a live test: http://ideone.com/Bu5aae

Answer 2

A suggestion:

SELECT FK, CONCAT(T1.F1,'',T2.F1) AS Result
FROM table T1, table T2
WHERE T1.FK = T2.FK