Good Example can be when incase of writing it after the end of your query if you have joins or where close :
select 'idPago','fecha','lead','idAlumno','idTipoPago','idGpo'
union all
(select id_control_pagos, fecha, lead, id_alumno, id_concepto_pago, id_Gpo,id_Taller,
id_docente, Pagoimporte, NoFactura, FacturaImporte, Mensualidad_No, FormaPago,
Observaciones from control_pagos
into outfile 'c:\\data.csv'
FIELDS TERMINATED BY ','
OPTIONALLY ENCLOSED BY '"'
LINES TERMINATED BY '\n');