Merge two rows in SQL

Aggregate functions may help you out here. Aggregate functions ignore NULLs (at least that's true on SQL Server, Oracle, and Jet/Access), so you could use a query like this (tested on SQL Server Express 2008 R2):

SELECT
    FK,
    MAX(Field1) AS Field1,
    MAX(Field2) AS Field2
FROM
    table1
GROUP BY
    FK;

I used MAX, but any aggregate which picks one value from among the GROUP BY rows should work.

Test data:

CREATE TABLE table1 (FK int, Field1 varchar(10), Field2 varchar(10));

INSERT INTO table1 VALUES (3, 'ABC', NULL);
INSERT INTO table1 VALUES (3, NULL, 'DEF');
INSERT INTO table1 VALUES (4, 'GHI', NULL);
INSERT INTO table1 VALUES (4, 'JKL', 'MNO');
INSERT INTO table1 VALUES (4, NULL, 'PQR');

Results:

FK  Field1  Field2
--  ------  ------
3   ABC     DEF
4   JKL     PQR

There are a few ways depending on some data rules that you have not included, but here is one way using what you gave.

SELECT
    t1.Field1,
    t2.Field2
FROM Table1 t1
    LEFT JOIN Table1 t2 ON t1.FK = t2.FK AND t2.Field1 IS NULL

Another way:

SELECT
    t1.Field1,
    (SELECT Field2 FROM Table2 t2 WHERE t2.FK = t1.FK AND Field1 IS NULL) AS Field2
FROM Table1 t1

I had a similar problem. The difference was that I needed far more control over what I was returning so I ended up with an simple clear but rather long query. Here is a simplified version of it based on your example.

select main.id, Field1_Q.Field1, Field2_Q.Field2
from 
(
    select distinct id
    from Table1
)as main
left outer join (
    select id, max(Field1)
    from Table1
    where Field1 is not null
    group by id
) as Field1_Q on main.id = Field1_Q.id
left outer join (
    select id, max(Field2)
    from Table1
    where Field2 is not null
    group by id
) as Field2_Q on main.id = Field2_Q.id 
;

The trick here is that the first select 'main' selects the rows to display. Then you have one select per field. What is being joined on should be all of the same values returned by the 'main' query.

Be warned, those other queries need to return only one row per id or you will be ignoring data

Merge two rows in SQL

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