TSQL Update statement with Join - sql

Question

I have two tables with a parent/child relationship. I want to update the parent with data from the child. However assuming there are 2 children I would like to be able to pick which child is used for the update depending on some other column from the child.
Here is what I have so far:
Parent: #test
Child: #exdat
Expected outcome, result of parent after update should contain only Capital letters. I want to update the parent with some data from the child, but if more than one exists I'd rather choose a child with a dimp of p1 over p2, p2 over p3 and p3 over p4
DECLARE #test TABLE
(
id int,
val char(1)
);
DECLARE #exdat TABLE
(
id int,
dval char(1),
dimp char(2)
);
INSERT INTO #test (id,val)
SELECT 1,'a'
UNION ALL SELECT 2,'b'
UNION ALL SELECT 3,'c'
UNION ALL SELECT 4,'d'
UNION ALL SELECT 5,'e'
UNION ALL SELECT 6,'f'
UNION ALL SELECT 7,'g'
;
INSERT INTO #exdat (id,dval,dimp)
SELECT 1,'A','p1'
UNION ALL SELECT 2,'B','p3'
UNION ALL SELECT 3,'C','p1'
UNION ALL SELECT 4,'D','p2'
UNION ALL SELECT 5,'E','p2'
UNION ALL SELECT 6,'F','p3'
UNION ALL SELECT 7,'w','p2'
UNION ALL SELECT 7,'g','p3'
UNION ALL SELECT 7,'G','p1'
UNION ALL SELECT 7,'z','p4'
;
UPDATE #test SET
val = e.dval
FROM
#test t
INNER JOIN #exdat e ON t.id = e.id
;
SELECT * FROM #test;
Result:
1 A
2 B
3 C
4 D
5 E
6 F
7 w <-- problem illustrated here
This "w" could have been any of the values w,g,G,z. So I am asking how can I prioritize the child selection based on some other column ?

Answer 1

What you want to do instead of a join is a subquery. Something like this:
UPDATE
a
SET
a.val = ISNULL((
SELECT TOP 1 x.dval
FROM #exdat x
WHERE x.id = a.id
ORDER BY x.magic_field -- <- here's how you specify precedence
), 'ReasonableDefault')
FROM
#test a

Answer 2

Trying using a CROSS APPLY with your update. The example below orders by the #extdat.dimp value:
DECLARE #test TABLE
(
id int,
val char(1)
);
DECLARE #exdat TABLE
(
id int,
dval char(1),
dimp char(2)
);
INSERT INTO #test (id,val)
SELECT 1,'a'
UNION ALL SELECT 2,'b'
UNION ALL SELECT 3,'c'
UNION ALL SELECT 4,'d'
UNION ALL SELECT 5,'e'
UNION ALL SELECT 6,'f'
UNION ALL SELECT 7,'g'
;
INSERT INTO #exdat (id,dval,dimp)
SELECT 1,'A','p1'
UNION ALL SELECT 2,'B','p3'
UNION ALL SELECT 3,'C','p1'
UNION ALL SELECT 4,'D','p2'
UNION ALL SELECT 5,'E','p2'
UNION ALL SELECT 6,'F','p3'
UNION ALL SELECT 7,'w','p2'
UNION ALL SELECT 7,'g','p3'
UNION ALL SELECT 7,'G','p1'
UNION ALL SELECT 7,'z','p4'
;
UPDATE #test
SET
t.val = e.dval
FROM #test as t
CROSS APPLY
(
SELECT TOP(1) * FROM #exdat as cae
WHERE t.id = cae.id
ORDER BY cae.dimp
) as e
;
SELECT * FROM #test;
If you use this method your results would look like this:
Result:
1 A
2 B
3 C
4 D
5 E
6 F
7 G
You can modify this by changing the order by in your CROSS APPLY

Answer 3

UPDATE #test
SET t.val = e.dval
FROM #test t
JOIN #exdat e
ON t.id = e.id
JOIN
( SELECT id
, MIN(dimp) AS dimp --orders by dimp ascending
FROM #exdat
WHERE dval = UPPER(dval) --keeps only rows with capital letters in dval
GROUP BY id
) AS g
ON e.id = g.id
AND e.dimp = g.dimp

Answer 4

I think using the ranking functions can be put to good use here.
In this sample I used DENSE_RANK to pick the highest dimp value by ORDER BY dimp and then looking for the one had the rank of one.
WITH cte
AS (SELECT Dense_rank() OVER (PARTITION BY id ORDER BY dimp) AS foo,
*
FROM #exdat)
UPDATE #test
SET val = e.dval
FROM #test t
INNER JOIN cte e
ON t.id = e.id
WHERE foo = 1;