Queries :: Update Query (table To Table) Not Updating All Records

I'm using an UPDATE query to update records in one table (tblMain) from another table (tblTemp)

Here is my SQL :

Quote:

UPDATE [tblMain]
INNER JOIN [tblTemp] ON [tblMain].[MainField1] = [tblTemp].[TempField1]
SET [tblMain].[MainField2] = [tblTemp].[TempField2];

I only want to update the records in tblMain which have a corresponding record in tblTemp (linked by MainField1 / TempField1)

If any record doesn't appear in tblTemp, I want tblMain to retain the existing value for that record.

However, it appears that in such situations, the record in tblMain has it's MainField2 value set to null / ZLS.

I've tried using LEFT JOIN and RIGHT JOIN and also tried WHERE clauses but the result is the same every time.