[Typo3] Blob Fe_users Fe-usergroups

[Alain]

Good morning,

In the typo3 mysql database, the fe_users table contains a blob field 
(usergroups) where you can find all the groups to which the user belongs.

Can someone tell me how this blob field is calculated ? When I export 
the data with phpMyAdmin under the CSV format, I can read the list for 
the  usergroups field (fe : (1,5,12)) but when I export under the SQL 
format I can't see how it is calculated.

As I have 5000 users to update, I would like to avoid doing it manually.

I thank you for your help.

Regards.

Alain Reymond