SQL Server 2016 - Variable showing NULL - sql-server-2016

Question

We have a scalar function in our application as below
CREATE function dbo.SCMGetEnvProfileValueFn
(#HierarchyCode varchar(255), -- Usually the subsystem code
#Code varchar(50), -- The Code to find
#Default varchar(255) -- If not found, return this default
)
RETURNS varchar(255)
AS
BEGIN
DECLARE #Value as varchar(255)
SELECT #Value = (SELECT TOP(1) Value FROM HVCEnvProfile
WHERE HierarchyCode = #HierarchyCode
AND Code = #Code)
RETURN ISNULL (#Value, #Default)
END
We converted this function to Table function
CREATE FUNCTION SCMGetEnvProfileValueTblFn
(#HierarchyCode varchar(255), -- Usually the subsystem code
#Code varchar(50), -- The Code to find
#Default varchar(255) -- If not found, return this default
)
RETURNS TABLE
AS
RETURN
(SELECT TOP(1) ISNULL (Value, #Default) AS value
FROM HVCEnvProfile
WHERE HierarchyCode = #HierarchyCode AND Code = #Code)
Below 2 statements shows different output. We do not have a column in the table HVCEnvProfile for this entry. Why the variable #Value is showing NULL when there is no row in the table.
SELECT value
FROM dbo.SCMGetEnvProfileValueTblFn('Registration', 'AdmitDtmEffectsLocationHistory', 'TRUE')
SELECT dbo.SCMGetEnvProfileValueFn('Registration', 'AdmitDtmEffectsLocationHistory', 'TRUE')

Answer 1

If a query returns no rows, then no rows will be returned, wrapping an ISNULL won't change that. Example:
SELECT ISNULL(V.C,0)
FROM (VALUES(1),(2),(3),(4))V(C)
WHERE V.C = 5;
Notice this does not return 0, but nothing.
You need to wrap the entire query in an ISNULL.
CREATE FUNCTION SCMGetEnvProfileValueTblFn (#HierarchyCode varchar(255), -- Usually the subsystem code
#Code varchar(50), -- The Code to find
#Default varchar(255) -- If not found, return this default
)
RETURNS table
AS
RETURN
SELECT ISNULL((SELECT TOP (1) [Value]
FROM HVCEnvProfile
WHERE HierarchyCode = #HierarchyCode
AND Code = #Code
ORDER BY SomeColumn), #Default) AS [Value]; --Don' forgot to change the value of SomeColumn
Don't forget, as well, you need an ORDER BY when using a TOP unless you're "happy" with inconsistent results (which I doubt), so i have added one that you will need to amend.