Not sure what you need, but you can manipulate fields using javacript, they are no different from any other html. To manipulate things on server side you can use code pack  https://www.seblod.com/store/extensions/1854

Also there are plugins to manipulate  sql, e.g.

https://www.seblod.com/store/extensions/19157

https://www.seblod.com/store/extensions/922

https://www.seblod.com/store/extensions/19162

Where can I find tutorials and examples on how to use Sql pack (use as field).

My question is:

I have a Content Type Form with

1)  field select dynamic: to select value from a table 'Products' ( id, name, price)

2) field text: quantity to sell

3) field Sql Query (contains total) = product price * quantity. But first I must  recover the price of the product (selected in field select dynamci), and then multiply it with the quantity: how can I do this?

thanks.

You don't need any custom script for that, Seblod has built in computation

I saw "computation rules" but it works with fields already in the form.

In my case I have in the form "Product id": through  id I have to recover the price and then multiply it by the quantity field.

I do not know how to recover the 'Price" in table Product through the ID  that I have in field 'product id' select dynamic.

thanks

You need

https://www.seblod.com/store/extensions/2997

With this field you can have product in first select, then pick up price in the second, based on the selection in the first field. Then you can use calculation to multiply quantity and price from second field.

And  answer to the actual topic of this thread, if anyone else will be searching for it:

https://www.seblod.com/resources/extensions/plug-ins/sql-pack