Need help generating select fields

[JJR512]

I'm having some trouble writing a script that generates four select fields.

There are four custom tables in the database (T1, T2, T3, T4). The first table has two columns (T1_ID and T1_Name). The next three tables each have three columns, like T2_ID, T2_Name, and T1_ID. Note that the third column relates to the ID from the table before it.

Each select field should be populated with options that are made from the rows in one of the tables. For example, the first table would have options with values from T1_ID and titles (the part in between the option tags) from T1_Name.

When a user selects something from the first select field, the option value selected should be used to narrow down the choices in the second select field to only those in the second table where the third column matches the value of the option selected in the first select field. And so on with the third and fourth fields, where each field is used to narrow down the choices in the next field.

The second, third, and fourth fields should initially be blank, and not have any options in them to select.

If a user selects something from the first, then the second, then the third field, for example, then changes the option selected in the second field, the third field should re-update to have the appropriate corresponding set of narrowed-down options.

I'm having trouble doing this myself. I hope I've explained it all clearly enough for someone to show me how this would be written.