How to do conditional update on columns using CTE? - sql

Question

I have a table CUST with following layout. There are no constraints. I do see that one ChildID has more than one ParentID associated with it. (Please see the records for ChildID = 115)
Here is what I need -
Wherever one child has more than 1 parent, I want to update those ParentID and ParentName with the ParentID and ParentName which has max match_per. So in the below image, I want ParentID 1111 and ParentName LEE YOUNG WOOK to update all records where ChildId = 115 (since the match_per 0.96 is maximum within the given set). In case there are two parents with equal max match_per, then I want to pick any 1 one of them.
I know it is possible using CTE but I don't know how to update CTE. Can anybody help?

Answer 1

One way of doing it
WITH CTE1 AS
(
SELECT *,
CASE WHEN match_per =
MAX(match_per) OVER (PARTITION BY ChildId)
THEN CAST(ParentId AS CHAR(10)) + ParentName
END AS parentDetailsForMax
FROM CUST
), CTE2 AS
(
SELECT *,
MAX(parentDetailsForMax) OVER (PARTITION BY ChildId) AS maxParentDetailsForMax
FROM CTE1
)
UPDATE CTE2
SET ParentId = CAST(LEFT(maxParentDetailsForMax,10) AS int),
ParentName = SUBSTRING(maxParentDetailsForMax,10,8000)

Answer 2

Getting both the parent id and parent name is a bit tricky. I think the logic is easiest using cross apply:
with toupdate as (
select t.*, p.parentId as new_parentId, p.parentName as new_parentName
max(match_per) over (partition by childid) as max_match_per,
count(*) over (partition by childid) as numparents
from table t cross apply
(select top 1 p.*
from table p
where p.childid = t.childid
order by match_per desc
) p
)
update toupdate
set parentId = new_ParentId,
parentName = new_ParentName
where numparents > 1;
As a note: the fact that parent id and parent name are both stored in the table, potentially multiple times seems like a problem. I would expect to look up the name, given the id, to reduce data redundancy.

Answer 3

Try something like this?? The first CTE will get Max(match_per) for each ChildID. Then, the second will use the new MaxMatchPer to find what its corresponding ParentID should be.
; WITH CTE AS (
SELECT ChildID,MAX(match_per) AS MaxMatchPer
FROM tbl
GROUP BY ChildID
), CTE1 AS (
SELECT t.ParentID, c.ChildID
FROM tbl t
JOIN CTE c
ON c.ChildID = t.ChildID
AND c.MaxMatchPer = t.match_per
)
UPDATE t
SET ParentID = c.ParentID
FROM tbl t
LEFT JOIN CTE1 c
ON c.ChildID = t.ChildID
Also, this is poor normalization. You should not have ParentName nor ChildName in this table.