sql query: Get roles that do not belong to a user

You can solve it with LEFT JOIN

SQL - Left join

The left join brings all the rows of the table on the left in addition to those that satisfy the join condition. For those that do not satisfy it, add null values in the columns of the table on the right.

What interests you is to take into account only when the user is 'pedro' and in turn get the rows where there is no matcheo with the table ENLACE , so you can first get the links from pedro and then take into account only the tuples that user_id is null to obtain the roles that do not belong to pedro.

Solution:

select * from rol left join (select * from enlace where id_user='pedro') as enlacespedro 
on (enlacespedro.id_rol = rol.id_rol) 
where id_user is null;

I created a test case here: SQL demo roles :

Regards

By SQL I suggest the following:

Select * from Rol
WHERE 
   IdRol Not IN ( 
                  Select IdRol 
                  From Enlace 
                  Where 
                     IdUsr IN (Select IdUsr From Usuario WHERE Usuario='pedro')  
                )

I clarify that I'm using Usuario as the name of the column where you have the data compare, in your example you put that you insert pedro and then you look for pedro@ so apply it as it really corresponds.