updating column based off of parent information - sql-server

Question

I am a beginner at SQL Server and I have a question about how best to do this.
I have a table that looks like this:
ID Parent Level
1 NULL 0
2 1 1
3 1 1
4 2 2
5 2 2
6 3 2
7 2 2
8 5 4
9 4 3
10 6 3
11 6 3
As you can see, all the entries have a Parent and a Level and the database is organized in a tree structure. There are some entries where the Level is set incorrectly such as entry ID #8. The Parent of 8 is 5 and ID 5 has a level of 2 so the level of 8 should be 3 and not 4. There are many incorrect Level values in my table and I'm not sure how to fix this. So far I have this:
UPDATE myTable
SET level=level-1
FROM myTable
WHERE ???;
I am not sure how to fill in the WHERE part or whether this is the best way to do this. Any suggestions are gladly appreciated.

Answer 1

This will show you the rows that have issues.
select
a.id,
a.level,
b.level as parentlevel
from
tablename a
join tablename b on a.parent = b.id
where
a.level <> b.level+1

Answer 2

If you are using SQL Server 2005 or SQL Server 2008, then you can use a recursive CTE (common table expression). The books online article is pretty straight forward, but here's how you can do it with your code.
-- Create temporary table and insert values
CREATE TABLE dbo.ctetest (childid int primary key not null, parentid int null, level int null);
INSERT INTO dbo.ctetest (childid, parentid) SELECT 1, NULL;
INSERT INTO dbo.ctetest (childid, parentid) SELECT 2, 1;
INSERT INTO dbo.ctetest (childid, parentid) SELECT 3, 1;
INSERT INTO dbo.ctetest (childid, parentid) SELECT 4, 2;
INSERT INTO dbo.ctetest (childid, parentid) SELECT 5, 2;
INSERT INTO dbo.ctetest (childid, parentid) SELECT 6, 3;
INSERT INTO dbo.ctetest (childid, parentid) SELECT 7, 2;
INSERT INTO dbo.ctetest (childid, parentid) SELECT 8, 5;
INSERT INTO dbo.ctetest (childid, parentid) SELECT 9, 4;
INSERT INTO dbo.ctetest (childid, parentid) SELECT 10, 6;
INSERT INTO dbo.ctetest (childid, parentid) SELECT 11, 6;
-- Update table with level data from recursive CTE
WITH recursivecte (childid, parentid, level)
AS
(SELECT childid
, parentid
, 'level' = 0
FROM dbo.ctetest
WHERE parentid IS NULL
UNION ALL
SELECT ct.childid
, ct.parentid
, 'level' = rc.level + 1
FROM dbo.ctetest ct
JOIN recursivecte rc
ON ct.parentid = rc.childid)
UPDATE ct
SET level = rc.level
FROM dbo.ctetest ct
JOIN recursivecte rc
ON ct.childid = rc.childid;
-- Verify results
SELECT *
FROM dbo.ctetest;
Here's the output from the above query:
Child ID Parent ID Level
1 NULL 0
2 1 1
3 1 1
4 2 2
5 2 2
6 3 2
7 2 2
8 5 3
9 4 3
10 6 3
11 6 3
Please note I tested the above code using SQL Server 2008. I'm assuming it will work in SQL Server 2005 since CTE's were introduced in 2005.