Using Developer Mode you can hover over any field to see the business object (model) details:

This tells you the Customer Payment Term field is called property_payment_term and that it is a many2one field that relates the res.partner model (the model that holds the basic Customer information) with the account.payment.term model.

Property fields like this are stored in the ir_property model.

More about these kinds of fields at https://doc.openerp.com/trunk/server/03_module_dev_02/#property-fields

Use this SQL

SELECT RP.NAME PARTNER, PT.NAME PAYMENT_TERM FROM IR_PROPERTY IP, RES_PARTNER RP, ACCOUNT_PAYMENT_TERM PT WHERE IP.NAME LIKE '%payment_term' AND CONCAT('res.partner,',RP.ID) = IP.RES_ID AND CONCAT('account.payment.term,',PT.ID) = IP.VALUE_REFERENCE

ACCOUNT_PAYMENT_TERM

id (ex 1)
name (ex 'Immediate payment')

RES_PARTNER

id = the ide of the partner

IR_PROPERTY

name="property payment term"
value_reference="account.payment.term," + the id of the account_payment__term (ex 1)
res_id = "res_partner," + the id the partner