Greatest-n-per-group problem

Question

Hi folks.
Say I have the following (simplified):
[Parent] id, name, address
[Child1] id, parentId, lastUpdatedBy, lastUpdatedOn
[Child2] id, parentId, lastUpdatedBy, lastUpdatedOn
[Child3] id, parentId, lastUpdatedBy, lastUpdatedOn
I want to do a select of the parent and the last updated details of whichever child was most recently updated.
SELECT
parent.ID,
child1.lastUpdatedOn,
child2.lastUpdatedOn,
child3.lastUpdatedOn,
child1.lastUpdatedBy,
child2.lastUpdatedBy,
child3.lastUpdatedBy
FROM PARENT parent
LEFT OUTER JOIN CHILD1 child1
ON parent.id = child1.parentId
LEFT OUTER JOIN CHILD1 child2
ON parent.id = child2.parentId
LEFT OUTER JOIN CHILD1 child3
ON parent.id = child3.parentId
AND NOT EXISTS (
SELECT * FROM CHILD1 child1_b
WHERE child1_b.parentId = child1.parentId
AND child1_b.lastUpdatedOn> child1.lastUpdatedOn
AND NOT EXISTS (
SELECT * FROM CHILD1 child2_b
WHERE child2_b.parentId = child2.parentId
AND child2_b.lastUpdatedOn> child2.lastUpdatedOn
AND NOT EXISTS (
SELECT * FROM CHILD1 child3_b
WHERE child3_b.parentId = child3.parentId
AND child3_b.lastUpdatedOn> child3.lastUpdatedOn
So my final record contains the parent and the lastUpdatedDetails for each child type.
parent.ID, lastUpdatedOn_1, lastUpdatedOn_2, lastUpdatedOn_3, lastUpdatedBy_1, lastUpdatedBy_2, lastUpdatedBy_3
I now want to get the most recent lastUpdatedBy and lastUpdatedOn across the three child types.
I can do:
GREATEST(child1.lastUpdatedOn, child2.lastUpdatedOn, child3.lastUpdatedOn) as lastUpdatedOn
but I can not do the same (as far as I am aware) with the lastUpdatedBy records.
Does anyone know how I might achieve this?
If only there was a function like GREATEST that could take a list of values to compare but also output values. So the result of greatest is not the comparison value but rather some associated values.
GREATEST(PAIR(INPUT, OUTPUT)...) that could allow me to do:
GREATEST(PAIR(child1.lastUpdatedOn, child1.lastUpdatedBy), PAIR(child2.lastUpdatedOn, child2.lastUpdatedBy), PAIR(child3.lastUpdatedOn, child3.lastUpdatedBy)) as lastUpdatedBy
Any help greatly appreciated.
Thanks
Chris

Answer 1

Hi,
You can do something like this:
WITH     got_r_num     AS
     SELECT     p.id
     ,     c.lastupdatedon
     ,     c.lastupdatedby
     ,     ROW_NUMBER () OVER ( PARTITION BY  p.id
                               ORDER BY        lastupdatedon  DESC
                         ,                lastupdatedby     -- See note below
                       )      AS r_num
     FROM             parent     p
     LEFT OUTER JOIN  child     c  ON  p.id  = c.parentid
SELECT       id
,       lastupdatedon
,       lastupdatedby
FROM       got_r_num
WHERE       r_num     = 1
ORDER BY  id
;If the combination (parentid, lastupdatedon) is not unique in child, then this considers a row with the earlier lastupdatedby to be later other rows with the same parentid and lastudpdatedon. Depending on your data and your reequirements, you may need to change the analytic ORDER BY clause, and/or use RANK instead of ROW_NUMBER.
I hope this answers your question.
If not, post a little sample data (CREATE TABLE and INSERT statements, relevant columns only) for all the tables involved, and the results you want from that data.
Explain, using specific examples, how you get those results from that data.
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