How can I change NULL to 0 when getting a single value from a SQL function?

SELECT 0+COALESCE(SUM(Price),0) AS TotalPrice
FROM Inventory
WHERE (DateAdded BETWEEN @StartDate AND @EndDate)

SELECT COALESCE(
    (SELECT SUM(Price) AS TotalPrice 
    FROM Inventory
    WHERE (DateAdded BETWEEN @StartDate AND @EndDate))
    , 0)

If the table has rows in the response it returns the SUM(Price). If the SUM is NULL or there are no rows it will return 0.

Putting COALESCE(SUM(Price), 0) does NOT work in MSSQL if no rows are found.

Most database servers have a COALESCE function, which will return the first argument that is non-null, so the following should do what you want:

SELECT COALESCE(SUM(Price),0) AS TotalPrice
FROM Inventory
WHERE (DateAdded BETWEEN @StartDate AND @EndDate)

Since there seems to be a lot of discussion about

COALESCE/ISNULL will still return NULL if no rows match, try this query you can copy-and-paste into SQL Server directly as-is:

SELECT coalesce(SUM(column_id),0) AS TotalPrice 
FROM sys.columns
WHERE (object_id BETWEEN -1 AND -2)

Note that the where clause excludes all the rows from sys.columns from consideration, but the 'sum' operator still results in a single row being returned that is null, which coalesce fixes to be a single row with a 0.

You can use ISNULL().

SELECT ISNULL(SUM(Price), 0) AS TotalPrice 
FROM Inventory
WHERE (DateAdded BETWEEN @StartDate AND @EndDate)

That should do the trick.