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ИЗПОЛЗВАЙТЕ агрегатната функция SUM() с GROUP BY :

select ts.name as my_name, ss.step_number, p.specs, SUM(p.price),  ssp.class_id
from optional_system_step  as ss
join system as s on s.system_id=ss.system_id
join category_description  as cd on cd.category_id=ss.category_id
join optional_system_step_product as  ssp on ss.system_step_id=ssp.system_step_id
join product as p on  p.product_id=ssp.product_id
join product_description as pd on  pd.product_id=p.product_id
join template_step as ts on  (ts.template_id=s.optional_template_id and ts.step_number=ss.step_number)
where s.system_id = '15' 
GROUP BY ts.name, ss.step_number, p.spects, ssp.class_id
order by ss.step_number, ssp.class_id;