Creating indexes based on specific SQL queries

The parameters got nothing to do with the indexes. The indexes are based on the columns that you use more in the WHERE clause. In the first table you use more: product_id customer_id order_date In the second table you use more: UnitsInstock ProductName SupplierID

you can add an index on this columns but just remember that an index is good for querying the table but are bad for inserting and updating in the table (the insert/update is slower).

On other hand the @field are just parameters (you can think of it as variables) In some other part of the script this parameters are getting the value.