You should never really interact with the SQL tables directly as it bypasses all of NAV's business logic which is stored in the objects.
What does the number you have look like? INVX? PO? Can you post the number or a sample of it here? Then we can point you in the right direction to access the details within NAV.
Update (6/23) based on the comments below.
The invoice credits will probably be stored as Sales Invoice Lines associated with the Sales Invoice Header. The following SQL query should get you all the lines associated with the header:
SELECT *
FROM [NAV].[dbo].[CompanyName$Sales Invoice Line]
WHERE [Document No_] = 'INVX-0000001'
GO
Obviously change NAV to your DB name and CompanyName to the appropriate company name within NAV.
Did this work?