self join- how to use the aggregate functions

Question

self join- how to use the aggregate functions

select  a.tablename,
b.TableName,b.UserName from Employee a inner join 
Employee b on  a.ColumnValue=b.ColumnValue
 and  and a.TableName <> b.TableName  and 
 a.UserName=b.UserName 

and also to check whether the same user has count of records i.e Employee a = count of records of Employee b.

how to add count function over here