[TYPO3-mvc] query for match two users of same usergroup

[Domi]

Hello Toke,

I try with pure SQL statement (or extbase query functions) to match 
usergroups from my data model MailEntry.

MailEntry
   -fromUser (Tx_Extbase_Domain_Model_FrontendUser)
   -toUser  (Tx_Extbase_Domain_Model_FrontendUser)

Now I want to get all MailEntries, where the fromUser->Usergroup is 
identical with toUser->Usergroup.

My current problem is, that I am not sure how to solve it, because 
fe_users->usergroup is a comma separated list.

Is this possible to archive with SQL or do I need to walk through the 
array with PHP?

Would be great to get some thoughts,

Cheers
Dominic