SQL Server function error with if condition - sql-server

Question

I'm still trying to learn an SQL, i made a mistakes but i already search on the internet about the if statements...
I tried to create a simple function to check if the parameter is match the condition..
I have 2 parameters, that will be inputted manually by the users, but i got an error saying
Msg 156, Level 15, State 1, Procedure ufn_calculatebonus, Line 4 [Batch Start Line 1] Incorrect syntax near the keyword 'IF'. Msg 178, Level 15, State 1, Procedure ufn_calculatebonus, Line 10 [Batch Start Line 1] A RETURN statement with a return value cannot be used in this context.
This is the code i've tried to create, i thought i make the IF condition right? After the IF , should have a BEGIN and statements, closing it with END. ??
CREATE FUNCTION Sales.ufn_calculatebonus (#CompanyRevenue int, #OptionalParameter varchar(60))
RETURNS TABLE
AS
IF #CompanyRevenue > 10
BEGIN
SELECT 'INPUT CANNOT BE BIGGER THAN 1000'
END
ELSE
BEGIN
RETURN
(
SELECT * FROM SALES.CUSTBALANCE2 where region=#OptionalParameter or name=#OptionalParameter
);
END
GO

Answer 1

There are 2 distinct types of T-SQL table-valued functions, inline TVF and multi-statement TVF. The body of an inline TVF consists of only of a RETURNS statement with a query specification (not to be confused with a RETURN statement).
Your attempt at a multi-statement table-valued function is invalid for the reasons below.
The body of a multi-statement TVF must be surrounded with BEGIN...END.
A table variable with the returned schema must be declared in the function header.
One cannot return query results with a RETURN statement. Instead, insert results into the table variable declared in the function header end specify RETURN at the end of the function to return the results.
Below is an example of the remediated code. Importantly, note the use of an explicit column list throughout. Don't SELECT * in production code.
CREATE OR ALTER FUNCTION dbo.ufn_calculatebonus (#CompanyRevenue int, #OptionalParameter varchar(60))
RETURNS #BalanceInfo TABLE (CustomerID int, Balance int, ErrorMessage varchar(100))
AS
BEGIN
IF #CompanyRevenue > 10
BEGIN
INSERT INTO #BalanceInfo (ErrorMessage) VALUES('INPUT CANNOT BE BIGGER THAN 1000');
END
ELSE
BEGIN
INSERT INTO #BalanceInfo (CustomerID, Balance) --note ErrorMessage is omitted and will be NULL
SELECT CustomerID, Balance FROM SALES.CUSTBALANCE2 where region=#OptionalParameter or name=#OptionalParameter;
END;
RETURN;
END
GO
A stored procedure is a better fit than a than user-defined function if you need to raise custom errors for parameter validation. Here's a stored procedure example:
CREATE OR ALTER PROCEDURE dbo.usp_calculatebonus (#CompanyRevenue int, #OptionalParameter varchar(60))
AS
IF #CompanyRevenue > 10
BEGIN
THROW 50000, 'INPUT CANNOT BE BIGGER THAN 1000', 16;
END;
SELECT CustomerID, Balance FROM SALES.CUSTBALANCE2 where region=#OptionalParameter or name=#OptionalParameter;
GO