Implement a stored procedure with conditions and update - sql-server

Question

The objective here is to update the table: BOOK_LOANS, column: #Status to a string (See code below) when I execute the Stored procedure. When the loan time goes past the Due_date adds 1 #Warning to the table READERS. When #Warnings = 3, SET #Eligibility to 'NOT ELIGIBLE'. How can I write this, this syntax is a mess.
IF SELECT DATEDIFF(day, #Due_date, CONVERT(date, getdate())) = 5
BEGIN
UPDATE BOOK_LOANS
SET #Status = 'DELAYED'
RETURN;
END
IF DATEDIFF(day, #Due_date, CONVERT(date, getdate())) = 3
BEGIN
UPDATE BOOK_LOANS
SET #Status = 'URGENT RETURN'
RETURN;
END
IF DATEDIFF(day, #Due_date, CONVERT(date, getdate())) = -1
BEGIN
UPDATE BOOK_LOANS
SET #Status = 'OUTDATED'
UPDATE READERS
SET #Warnings = #Warnings + 1
RETURN;
END
IF (#Warnings > 3)
BEGIN
UPDATE READERS
SET #Eligibility = 'NOT ELIGIBLE'
END

Answer 1

I think here is what you are trying to do :
UPDATE BOOK_LOANS
SET
status = CASE DATEDIFF(day, Due_date, CONVERT(date, getdate()))
WHEN 5 THEN 'DELAYED'
WHEN 3 THEN 'URGENT RETURN'
WHEN -1 THEN 'OUTDATED'
ELSE status
END
, warning = CASE WHEN DATEDIFF(day, Due_date, CONVERT(date, getdate())) = -1
THEN warning + 1
ELSE warning
END
GO
UPDATE r
SET Eligibility = CASE WHEN l.warning > 3
THEN 'NOT ELIGIBLE'
ELSE Eligibility
END
FROM READERS r
JOIN BOOK_LOANS l
on l.reader_id = l.reader_is -- here I assume you need to link two tables based on the PK, FKs

Answer 2

Thank you so much for the time. After some time learning a lit bit more I came to this final code. For some reason I couldn't apply the code above but nevertheless helped me to reach my final code.
UPDATE book_loans
SET loan_status = CASE
WHEN Datediff(day, CONVERT(DATE, Getdate()), due_date) =
5 THEN
'RETURN SOON'
WHEN Datediff(day, CONVERT(DATE, Getdate()), due_date) =
3 THEN
'URGENT RETURN'
WHEN Datediff(day, CONVERT(DATE, Getdate()), due_date) <=
-1
THEN 'OUTDATED'
ELSE loan_status
END;
-- Create temp table
SELECT r.card_no,
b.due_date,
Warnings = 0
INTO #temp3
FROM readers AS r
JOIN book_loans AS b
ON r.card_no = b.card_no
SELECT *
FROM #temp3
-- Calculate warnings
UPDATE #temp3
SET warnings = CASE
WHEN Datediff(day, CONVERT(DATE, Getdate()), due_date) <= -1
THEN
warnings + 1
ELSE warnings
END;
UPDATE readers
SET readers.warnings = readers.warnings + p.total
FROM (SELECT card_no,
Sum(warnings) AS total
FROM #temp3
GROUP BY card_no) p
WHERE readers.card_no = p.card_no
UPDATE readers
SET eligibility = CASE
WHEN warnings > 3 THEN 'NOT ELIGIBLE'
ELSE eligibility
END