Why should you care?

Formatting or simple calculations might live in a function, but set-based expressions often beat scalar UDFs in hot queries.

See it live — copy this example

Run in SQL Server Management Studio (SSMS) or Azure Data Studio.

USE DataVerse;
CREATE OR ALTER FUNCTION dbo.fn_AddGst(@Amount DECIMAL(18,2), @Rate DECIMAL(5,2))
RETURNS DECIMAL(18,2)
AS
BEGIN
    RETURN CAST(@Amount * (1 + @Rate / 100.0) AS DECIMAL(18,2));
END
GO
SELECT OrderId, Amount, dbo.fn_AddGst(Amount, 18) AS AmountWithGst
FROM dbo.Orders;

What happened?

• fn_AddGst returns amount with GST percent applied.
• The SELECT shows per-order inclusive totals.
• For huge scans, consider computing inline instead.

Practice next

1. Create the function and run the SELECT.
2. Compare to Amount * 1.18 inline in a plan.
3. Avoid scalar UDFs inside JOIN conditions on big tables.
4. Call dbo.fn_AddGst(100, 18) alone.
5. Add a second rate parameter default via wrapper proc.

Remember

Scalar UDF → one return value. Watch per-row cost in big queries. Inline math when performance matters.

GST display helper

DataVerse invoice UI shows amount with GST via fn_AddGst.

Outcome: Consistent tax math; heavy batch jobs use set-based formulas.