Adding WHERE clause to outer query causes inner query to error - sql

Question

The below query worked until I added a WHERE clause.
SELECT
PersonTable.FullName,
View_PersonToHead.DirectorId
FROM PersonTable
LEFT JOIN View_PersonToDirector ON PersonTable.PersonId = View_PersonToDirector.PersonId
WHERE View_PersonToDirector.DirectorId = 12345 --No error if this line is removed
The error message is:
Invalid length parameter passed to the RIGHT function.
This leads me to believe that there was an error with how I wrote the View_PersonToDirector view. Something about adding the WHERE clause causes the query to be evaluated/optimized in a different way, exposing some issue.
Internals of View_PersonToDirector:
WITH items AS (
SELECT
PersonId,
0 AS [Level],
CAST(PersonId AS VARCHAR(255)) AS [Path]
FROM PersonTable
UNION ALL
SELECT
e.PersonId,
[Level] + 1,
CAST([Path] + ' < ' + CAST(e.PersonId AS VARCHAR(255)) AS VARCHAR(255))
FROM PersonTable e
INNER JOIN items itms ON itms.PersonId = e.ManagerId
)
SELECT
A.PersonId,
CASE
WHEN A.[Level] = 1
THEN A.PersonId
ELSE CAST(LEFT(A.PathToDirector, CHARINDEX(' ', A.PathToDirector)) AS INT)
END AS DirectorId
FROM (
SELECT
items.PersonId,
items.[Level],
RIGHT(items.[Path], LEN(items.[Path])-7) AS PathToDirector
FROM items
WHERE Path LIKE '1111 < %' --Id of director
) A
I suspect that having a WITH cte in the view causes the query optimizer to work differently, applying the WHERE filtering in a different order. Is this bad practice?

Answer 1

One of your problems is definitely in this code:
RIGHT(items.[Path], LEN(items.[Path])-7) AS PathToDirector
I appreciate that you think that this WHERE clause fixes the problem:
WHERE Path LIKE '1111 < %'
But it does not. The problem is that SQL Server does not guarantee the order of evaluation of expressions. This true even for subqueries, CTEs, and views. These can be evaluated in any order. In fact, SQL Server sometimes pushes the evaluation to the node that reads from the table -- which is why you get an error.
Personally, I think this is a bug in SQL Server. Those powers that be do not agree. Here are two solutions:
(CASE WHEN Path LIKE '1111 < %' THEN RIGHT(items.[Path], LEN(items.[Path])-7) END) AS PathToDirector
Or:
(CASE WHEN Path LIKE '1111 < %' THEN RIGHT(items.[Path], LEN(items.[Path] + '1234567')-7) END AS PathToDirector
You may have the same problem with the CHARINDEX().

Answer 2

The use of a where condition related to column from left join table generate and implicit inner join
In this case add the condition to the ON clause for let the left join work
SELECT
PersonTable.FullName,
View_PersonToHead.DirectorId
FROM PersonTable
LEFT JOIN View_PersonToDirector ON PersonTable.PersonId = View_PersonToDirector.PersonId
AND View_PersonToDirector.DirectorId = 12345
for the length problem try use conditional eg using case
SELECT
items.PersonId,
items.[Level],
case when LEN(items.[Path]) > 7 then RIGHT(items.[Path], LEN(items.[Path])-7)
ELSE items.[Path] END AS PathToDirector
FROM items

Answer 3

The base part of your rCTE has:
CAST(PersonId AS VARCHAR(255)) AS [Path]
Which means base rows of the rCTE would contain values such as 1234 which are shorter than 7 characters and cause RIGHT(x, LEN(x) - 7) to fail. The condition:
RIGHT(items.[Path], LEN(items.[Path])-7) AS PathToDirector
Could be safely written as:
SUBSTRING(items.[Path], NULLIF(CHARINDEX(' < ', items.[Path]), 0) + 3, LEN(items.[Path]))