sql select into select in function - sql

Question

When I try to alter the function below I get the following error message:
Only one expression can be specified in the select list when the
subquery is not introduced with EXISTS.
I guess it is probably because of select into select. But why does this select into select work separately ( not in function ) but not in function.
ALTER FUNCTION [dbo].[Getcurrentexchangerate] (#CurrencyFromId INT,
#CurrencyToId INT)
returns DECIMAL(13, 10)
AS
BEGIN
DECLARE #rate DECIMAL (13, 10)
DECLARE #dw INT
SET #dw = (SELECT Datepart(dw, Getdate()))
IF( #dw != 2 ) -- Monday
BEGIN
SET #rate = (SELECT TOP (1) [rate]
FROM currencyconversionrate
WHERE [currencyfromid] = #CurrencyFromId
AND [currencytoid] = #CurrencyToId
ORDER BY id DESC)
END
ELSE
BEGIN
SET #rate = (SELECT *
FROM (SELECT TOP(2) Row_number()
OVER (
ORDER BY id DESC) AS
rownumber,
rate
FROM currencyconversionrate
WHERE ( [currencyfromid] = 2
AND [currencytoid] = 5 )
ORDER BY id DESC) AS Rate
WHERE rownumber = 2)
END
IF( #rate IS NULL )
BEGIN
SET #rate = 1
END
RETURN #rate
END

Answer 1

See your "else" part
SET #rate = (SELECT *
FROM (SELECT TOP(2) Row_number()
OVER (
ORDER BY id DESC) AS
rownumber,
rate
FROM currencyconversionrate
WHERE ( [currencyfromid] = 2
AND [currencytoid] = 5 )
ORDER BY id DESC) AS Rate
WHERE rownumber = 2)
You're trying to select all fields from currencyconversionrate table, you can't do that, or do you want to select "RATE" column only?
Try changing to below:
SET #rate = (SELECT rate
FROM (SELECT TOP(1) Row_number()
OVER (
ORDER BY id DESC) AS
rownumber,
rate
FROM currencyconversionrate
WHERE ( [currencyfromid] = 2
AND [currencytoid] = 5 )
ORDER BY id DESC) AS Rate
WHERE rownumber = 2)