mira si este SQL va mejor
Código SQL
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SELECT cob.id_obra,cob.nombre_obra, bf.nom_beneficiario,SUM(pg.importe_pago) AS total_pagos,SUM(dc.importe_desc) AS total_descuentos
FROM obra AS cob
left join beneficiario AS bf on cob.id_obra = bf.id_obra
left join pagos AS pg on bf.id_beneficiario= pg.id_beneficiario
left join descuentos AS dc on bf.id_beneficiario= dc.id_beneficiario
WHERE
cob.id_obra= :id_obra