[V4MD] help for check field values and link more tables

[info at vallemediatime.com]

Hi Ruslan,
two question:

1. is possible to check the value of two strings field and if the two value already exist create a Error Msg?
eg.

Person     Courses     Registration_Date
----------------------------------------------------------
Smith        Math          26-09-2005
Smith        Music        26-09-2005
Brown        Math           27-09-2005
Smith        Math          > ERROR


2. More big question
In the previous example I think that it is better create a third linked table... I need to add the date of registration for every Student-Course also: in this case I need in your big suggestion for understand the better way to make it. 

In my project I have all Students into a T1 with a CodeID that identifying all persons of the school (eg. Students = 1, Teachers = 2 and so on). I'm tryng with API and binary link, but it's not simple to find right values from all three tables as: T3 linked to T1 and T3 linked at T2. 
I hope in a example... the better way for understand Valentina :-)

T1
Person   CodeID
------------------------
Smith       1       
Brown      1
Cher        2
...

T2
Courses
------------
Math 
Music
...

T3
Date of registration
----------------------------
How is better to insert this?

eg If I will find all Students that have registration to Math in 26-09-2005 date?



Thanks
Paolo