Validate if value exists before insert row - sql

Question

I have a query where I send a TableType who have columns EmpKey and TaskId like:
#AssignNotificationTableType [dbo].[udf_TaskNotification] READONLY
INSERT INTO [TaskNotification] ([TaskId], [EmpKey])
SELECT
[ANT].[TaskId], [E].[EmpKey]
FROM
#AssignNotificationTableType AS [ANT]
INNER JOIN
[Employee] AS [E] ON [ANT].[EmpGuid] = [E].[EmpGuid]
So my table looks like this:
+--------------------------------------+--------------------------------------+--------+
| TaskNotificationId | TaskId | EmpKey |
+--------------------------------------+--------------------------------------+--------+
| EEE3D3F8-F190-E811-841F-C81F66DACA6A | D0440DEB-404C-4006-870F-E95BFFA840E0 | 44 |
| EFE3D3F8-F190-E811-841F-C81F66DACA6A | D0440DEB-404C-4006-870F-E95BFFA840E0 | 49 |
+--------------------------------------+--------------------------------------+--------+
As you can see two items have same TaskId but different Empkey, so suppose if I send again same TaskId D0440DEB-404C-4006-870F-E95BFFA840E0 I want to insert only row only if EmpKey does not exist in that TaskId
So if I send something like:
+--+--------------------------------------+--------+
| | TaskId | EmpKey |
+--+--------------------------------------+--------+
| | D0440DEB-404C-4006-870F-E95BFFA840E0 | 44 |
| | D0440DEB-404C-4006-870F-E95BFFA840E0 | 49 |
| | D0440DEB-404C-4006-870F-E95BFFA840E0 | 54 |
+--+--------------------------------------+--------+
It will only insert last row, because EmpKey 54 does not exist in that TaskId
I try to do in WHERE clause with NOT IN as:
INSERT INTO [TaskNotification] ([TaskId], [EmpKey])
SELECT
[ANT].[TaskId], [E].[EmpKey]
FROM
#AssignNotificationTableType AS [ANT]
INNER JOIN
[Employee] AS [E] ON [ANT].[EmpGuid] = [E].[EmpGuid]
WHERE
[E].[EmpKey] NOT IN (SELECT EmpKey
FROM [TaskNotification]
WHERE TaskId = (SELECT TaskId
FROM #AssignNotificationTableType))
But when I run it, it just don't insert anything. What am I doing wrong? Regards

Answer 1

Add the target table as a left join to the select statement:
INSERT INTO [TaskNotification]
(
[TaskId]
, [EmpKey]
)
SELECT
[ANT].[TaskId]
, [E].[EmpKey]
FROM #AssignNotificationTableType AS [ANT]
INNER JOIN [Employee] AS [E] ON [ANT].[EmpGuid] = [E].[EmpGuid]
LEFT JOIN [TaskNotification] AS [TN] ON [TN].[TaskId] = [ANT].[TaskId]
AND [TN].[EmpKey] = [E].[EmpKey]
WHERE [TN].[PK] IS NULL -- PK stands for the primary key column
-- (or first column in of a multiple columns pk)
Please note, however, this in a multithreaded environment such query might fail - For more information, read this SO post and Dan Guzman's blog post it links to.