Count difference between distinct and join over distinct - sql

Question

How do I count the number of distinct rows minus a join over those same distinct rows?
I'm writing after triggers where I need to raise an error if the user does not have rights to the rows submitted. I can do this in two statements but this seems inefficient.
DECLARE #AccessibleCount INT =
(
SELECT
COUNT(DISTINCT i.[ParentId])
FROM
inserted i
INNER JOIN [SuperSecret].[Parent] AS p ON
p.[Id] = i.[ParentId] AND
p.[LockedBy] = #UserId
);
DECLARE #ActualCount INT = (SELECT COUNT(DISTINCT [ParentId]) FROM inserted);
IF (#AccessibleCount <> #ActualCount)
BEGIN
RAISERROR(...);
ROLLBACK TRANSACTION;
END
For performance sake, it seems like I should use a subquery over the distinct inserted.ParentId for both counts. I tried the following but it resulted in "Invalid object name 'i'."
DECLARE #ActualMinusAccessible INT =
(
SELECT
COUNT(*)
-
(
SELECT
COUNT(*)
FROM
i
INNER JOIN [SuperSecret].[Parent] AS p ON
p.[Id] = i.[ParentId] AND
p.[LockedBy] = #UserId
)
FROM
(
SELECT DISTINCT [ParentId] FROM inserted
) AS i
);
IF (#ActualMinusAccessible <> 0)
BEGIN
RAISERROR (...);
ROLLBACK TRANSACTION;
END

Answer 1

If am not wrong you want to Raise Error if a [ParentId] is inserted which is not present in [SuperSecret].[Parent] table. Try changing your SQL query like this.
IF EXISTS (SELECT 1
FROM inserted i
WHERE NOT EXISTS (SELECT 1
FROM [SuperSecret].[Parent] a
WHERE i.[ParentId] = a.[ParentId] AND a.[LockedBy] = #UserId))
BEGIN
RAISERROR (...);
ROLLBACK TRANSACTION;
END
OR
IF (SELECT Count(DISTINCT [ParentId]) - (SELECT Count(DISTINCT i.[ParentId])
FROM inserted i
INNER JOIN [SuperSecret].[Parent] AS p
ON p.[Id] = i.[ParentId]
AND p.[LockedBy] = #UserId)
FROM inserted) <> 0
BEGIN
RAISERROR (...);
ROLLBACK TRANSACTION;
END