SQL Server trigger not acting as expected - sql

Question

I have these two tables:
CREATE TABLE [dbo].[Test_Table1]
(
[id] [int] IDENTITY(1,1) NOT NULL,
[f_id] [int] NULL
) ON [PRIMARY]
CREATE TABLE [dbo].[Test_Table2_Tbl]
(
[f_id] [int] IDENTITY(1,1) NOT NULL,
[text] [varchar](500) NULL
) ON [PRIMARY]
And a trigger:
CREATE TRIGGER [dbo].[trg_Test_Trigger_Delete]
ON [dbo].[Test_Table2_Tbl]
AFTER DELETE
AS
INSERT INTO Test_Table2_Tbl (text)
(SELECT id FROM deleted)
UPDATE Test_Table1_Tbl
SET f_id = NULL
WHERE f_id IN (SELECT id FROM deleted)
GO
The keen observer will realize that 'id' does not exist in 'deleted'.
SQL catches that error in the INSERT, but without the INSERT it will let you add the trigger without any complaints.
Why doesn't it catch that error? (Invalid column name 'id'.)
My second question is, why does the UPDATE statement update EVERY column to NULL when id is not found in deleted?
I'm just very confused and need so clarity on why every record matches that WHERE clause.

Answer 1

Your error in the UPDATE is because the query is treated as a correlated subquery:
UPDATE Test_Table1_Tbl
SET f_id = NULL
WHERE Test_Table1_Tbl.f_id IN (SELECT Test_Table1_Tbl.id FROM deleted d);
id doesn't resolve in deleted, so SQL looks out at the next level.
These are the scoping rules for subqueries. That is why you should always use qualified table names when you have subqueries:
UPDATE Test_Table1_Tbl
SET f_id = NULL
WHERE Test_Table1_Tbl.f_id IN (SELECT d.id FROM deleted d);