SQL Script Needs improvements, Could use some tips - sql

Question

I recently got a test to figure out how to optimize this SQL Script, I have never made a script, and only create queries to get data at my work. This is the Script.
The tables have valid data and joins, the #GUID is already declared and has NVARCHAR values.
This is the SQL script
I am pretty new to this, I need to optimize this script, and from what I know, its better to Join the query instead of using Sub-queries. (correct me if I am wrong)
From What I understand from this script, it is asking information from 2 different Databases
CUSTOMER_CARDNUMBER and PROFILE
I can't see the information in these Fake databases, but that is not part of this test.
In these databases it is asking for multiple information GENERATED_CARDNUMBER, and CUSTOMER_CARDNUMBER with the values = #GUID (which I don't completely understand)
IT WIll Begin a Update if the REVISION_Status = A, if GUID = #GUID what I understand this Script is looking for in both databases. and If it does not find it, it will set a REVISION_STATUS with a Date if the query does not find GUID = #GUID; ( Please correct me if wrong)
I could use help, in improving this script, how and why.. even if there are little mistakes.
Thanks!!
IF EXISTS
(
SELECT *
FROM CUSTOMER_CARDNUMBER
WHERE GENERATED_CARDNUMBER IN
(
SELECT CARDNUMBER
FROM Profile
WHERE Cardnumber IN
(
SELECT GENERATED_CARDNUMBER
FROM CUSTOMER_CARDNUMBER
WHERE CUSTOMER_CARDNUMBER = #GUID
)
)
)
BEGIN
UPDATE GUID_ACTION
SET
REVISION_STATUS = 'A',
REVISION_DATE_CREATED = GETDATE()
WHERE GUID = #GUID;
END;
ELSE
BEGIN
UPDATE GUID_ACTION
SET
REVISION_STATUS = 'F',
REVISION_DATE_CREATED = GETDATE()
WHERE GUID = #GUID;
END;

Answer 1

``UPDATE GUID_ACTION
SET REVISION_STATUS =
CASE WHEN (0 < select count(*) from CUSTOMER_CARDNUMBERCUSTOMER_CARDNUMBER
inner join PROFILE
ON PROFILE.Cardnumber = CUSTOMER_CARDNUMBER.GENERATED_CARDNUMBER
AND PROFILE.Cardnumber = #GUID
)
THEN 'A'
ELSE 'F'
END;
,GUID_ACTION.REVISION_DATE_CREATED = GETDATE()
WHERE CUSTOMER_CARDNUMBER.GUID = #GUID;
This is a another more cleaner way, the query script with the subquery was difficult to understand with no insight of the tables or declared #GUID