Hola AL3XZ4ND3R, tendría que ser algo así:
Código SQL
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Select t1.Id, t1.Fecha, t1.Hora, t1.UserName
From Tabla t1 Left Join (Select UserName, Count(*) as Contador
From Tabla
Group By UserName) t2
On (t1.UserName=t2.UserName)
Order by Contador Desc
Saluditos