How to fill ParentID column in one update statement? - sql-server

Question

My table is like this:
ID Code ParentID
-------------------
1 A01 NULL
2 B83 NULL
3 H92 NULL
15 A013 NULL
23 A018 NULL
33 A01899 NULL
44 B8329 NULL
67 B83293 NULL
What I want is to update ParentID to match the ID of the parent code.
A01 is the parent for A013
A01 is the parent for A018
A018 is the parent for A01899
and so on.
You can see the length of A01 is 3 while the child A013 length is 4, the length of A018 is 4 and the child A01899 length is 6.
I can do that with multiple update statements and repeat that for each case.
UPDATE A
SET ParentID = B.ID
FROM Table A
INNER JOIN Table B ON A.Code like B.Code + '%'
WHERE LEN(A.Code) = 4 AND LEN(B.Code) = 3
But the question is how to do that in a single update statement?

Answer 1

You can first find all the relevant matches - similar to what you have - but also the length of the code it matched to. Then find the one with the longest matched code.
CREATE TABLE #TableA (ID int, Code varchar(10), ParentID int);
INSERT INTO #TableA (ID, Code, ParentID)
VALUES
(1 , 'A01' , NULL),
(2 , 'B83' , NULL),
(3 , 'H92' , NULL),
(15, 'A013' , NULL),
(23, 'A018' , NULL),
(33, 'A01899', NULL),
(44, 'B8329' , NULL),
(67, 'B83293', NULL);
WITH A AS
(SELECT TA.ID, TA.ParentID, TB.ID AS TB_ID,
ROW_NUMBER() OVER (PARTITION BY TA.ID ORDER BY TB.Len_Code DESC) AS rn
FROM #TableA TA
INNER JOIN
(SELECT ID, Code, LEN(CODE) AS Len_Code
FROM #TableA
) TB ON TA.Code LIKE TB.Code + '%'
WHERE TA.ID <> TB.ID
)
UPDATE A
SET A.ParentId = A.TB_ID
WHERE A.rn = 1;
Result
ID Code ParentID
1 A01 NULL
2 B83 NULL
3 H92 NULL
15 A013 1
23 A018 1
33 A01899 23
44 B8329 2
67 B83293 44

Answer 2

Just another option.
Significant Digits can be a risky business in the long run. It has been my experience that they tend to have a rather short shelf-life until an exception needs to be made.
In the example below, we allow for up to three characters in distance. We apply the closest first via the coalesce()
Just to be clear, Left Join D may not be necessary if Parents are within 1 or 2 characters. Conversely, this could be expanded if needed Left Join D ...
Example
Declare #YourTable Table ([ID] int,[Code] varchar(50),[ParentID] int)
Insert Into #YourTable Values
(1,'A01',NULL)
,(2,'B83',NULL)
,(3,'H92',NULL)
,(15,'A013',NULL)
,(23,'A018',NULL)
,(33,'A01899',NULL)
,(44,'B8329',NULL)
,(67,'B83293',NULL)
;with cte as (
Select A.*
,PtNr = coalesce(B.ID,C.ID,D.ID)
From #YourTable A
Left Join #YourTable B on left(A.[Code],len(A.[Code])-1)=B.[Code]
Left Join #YourTable C on left(A.[Code],len(A.[Code])-2)=C.[Code]
Left Join #YourTable D on left(A.[Code],len(A.[Code])-3)=D.[Code]
)
Update cte set ParentID=PtNr
Select * From #YourTable
The Update Table
ID Code ParentID
1 A01 NULL
2 B83 NULL
3 H92 NULL
15 A013 1
23 A018 1
33 A01899 23
44 B8329 2
67 B83293 44