Update on a table using inner join in a trigger - sql

Question

I have two tables in the database, they have a one-to-many relationship (Example: The table dbo.Tree [tree_id, tree_code] has many dbo.Fruits[id, name, father_tree_id, father_tree_code]) .
I'm trying to create a trigger for when the column codigo_arvore_pai of dbo.Frutos is updated or inserted into it, the column father_tree_code of dbo.Frutos is updated with the value corresponding to the tree_id of the dbo.Tree table. The condition for this is code_tree of dbo.Tree to be equal code_tree_father of dbo.Fruits.
CREATE TRIGGER [dbo].[tr_updateFruit] on [dbo].[Fruits]
AFTER INSERT, UPDATE
AS
IF (UPDATE(father_tree_code))
BEGIN
UPDATE dbo.Fruits
SET id_arvore_pai = A.id_arvore
FROM dbo.Fruits as obj
INNER JOIN dbo.Tree A ON obj.father_tree_code = A.tree_code
WHERE obj.Id IN (SELECT DISTINCT obj.Id FROM dbo.Fruits)
END;
What's wrong?

Answer 1

When the command to update the SQL server is executed, in fact, first the record is deleted and then the new record is inserted again with new changes, giving the illusion to the user that the editing has been done on the desired fields. But in fact, the update command is a two-step command that consists of a deletion and an insertion. First, you must specify the type of action or command executed. You can use the following code inside of the trigger for this reason:
DECLARE #WhatActionHappened as char(1);
SET #WhatActionHappened = (CASE WHEN EXISTS(SELECT * FROM INSERTED)
AND EXISTS(SELECT * FROM DELETED)
THEN 'U' -- Set to Updated.
WHEN EXISTS(SELECT * FROM INSERTED)
THEN 'I' -- Set to Insert.
WHEN EXISTS(SELECT * FROM DELETED)
THEN 'D' -- Set to Deleted.
ELSE NULL -- Skip. It may have been a "failed delete".
END)
After that, you can use the trigger as follows to insert and edit.
CREATE TRIGGER [dbo].[tr_updateFruit] on [dbo].[Fruits]
AFTER INSERT, UPDATE
AS
BEGIN
DECLARE #WhatActionHappened as char(1);
SET #WhatActionHappened = (CASE WHEN EXISTS(SELECT * FROM INSERTED)
AND EXISTS(SELECT * FROM DELETED)
THEN 'U' -- Set to Updated.
WHEN EXISTS(SELECT * FROM INSERTED)
THEN 'I' -- Set to Insert.
--WHEN EXISTS(SELECT * FROM DELETED)
--THEN 'D' -- Set to Deleted.
ELSE NULL -- Skip. It may have been a "failed delete".
END)
IF(#WhatActionHappened = 'U' OR #WhatActionHappened = 'I')
BEGIN
UPDATE dbo.Fruits
SET id_arvore_pai = A.id_arvore
FROM dbo.Fruits as obj
INNER JOIN dbo.Tree A ON obj.father_tree_code = A.tree_code
WHERE obj.Id IN (SELECT DISTINCT i.Id FROM Inserted i)
END
END