Serial numbering of subcategories within each category

I have a table Categories, containing fields

• CategoryName
• CategorySerial#
• CategoryDescription

. There is a second table Subcategories, containing fields

• SubcatName
• SubcatSerial#
• CatSerial#
• Data

The tables are linked by way of the fields CategorySerial# and the key field CatSerial#.

Problem I would like the serial numbers in SubcatSerial# to "restart" from 1 within each Category, rather than simply having a unique SubcatSerial# for each record in Subcategories.

Using automatic serial numbers in SubcatSerial# is clearly not the way to do this, but am not sure what will work. Thanks for your suggestions.