While creating a SQL query to extract Purchase Order information, the subquery to retrieve item number afiliated with the Purchase order returns the following error: ORA-01427: single-row subquery returns more than one row.

Below is the full SQL query:

SELECT
     distinct item.item_number              item_number

FROM
     po_headers_all pha
    ,fun_all_business_units_v fav
    ,po_buyers_val_v pbv_inner
    ,poz_suppliers_v psv
    ,po_lines_all pla
    ,egp_categories_vl egp
    ,hr_locations_all hla
    ,hr_locations_all hla2
    ,po_distributions_all pda
    ,po_line_locations_all plla
    ,gl_code_combinations gcc
    ,pjf_tasks_v pt
    ,poz_supplier_sites_all_m pss
    ,hr_organization_units hru
    ,(select pjf.name,pb.segment1,pb.project_id
      from pjf_projects_all_b pb, pjf_projects_all_tl pjf
      where  pjf.project_id = pb.project_id)
      project
    ,(select pha2.segment1 ,pla2.line_num, pla2.po_line_id,pha2.po_header_id
      from po_headers_all pha2 ,po_lines_all pla2
      where  pha2.po_header_id = pla2.po_header_id  
      and pha2.type_lookup_code(+) = 'BLANKET')
      agreement
    ,(select esiav.item_number,esiav.inventory_item_id
      FROM egp_system_items_b esiav,inv_org_parameters IOP
      WHERE ESIAV.organization_id = IOP.organization_id 
      AND IOP.organization_code='WSH_ITEMMSTR')
      item  
     /* Old logic- requisition number not getting populated 
            ,(select 
        prh.requisition_number
        ,prl.po_header_id
        ,prl.req_bu_id
        ,prl.po_line_id
        ,pf.full_name
        from 
        por_requisition_lines_all prl
        ,por_requisition_headers_all prh
        ,PER_PERSON_NAMES_F pf
        where
        prl.requisition_header_id = prh.requisition_header_id
        AND prl.requester_id= pf.person_id(+)
        AND pf.name_type='GLOBAL')
      rq */
WHERE 1=1
    and pha.po_header_id           = pla.po_header_id
    and pha.po_header_id           = pda.po_header_id (+)
    and pha.agent_id               = pbv_inner.person_id
    and pha.vendor_id              = psv.vendor_id
    and plla.ship_to_location_id   = hla.location_id(+) 
    and pha.req_bu_id              = fav.bu_id(+)
    and pha.vendor_site_id         = pss.vendor_site_id
    and pla.po_line_id             = pda.po_line_id(+)
    and pla.item_id                = item.inventory_item_id(+)
    and pla.category_id            = egp.category_id     
    and pla.po_line_id             = plla.po_line_id
    and pla.from_header_id         = agreement.po_header_id(+)  
    and pla.from_line_id           = agreement.po_line_id(+)
    and pda.deliver_to_location_id = hla2.location_id(+)  
    and pda.code_combination_id    = gcc.code_combination_id(+)
    and pda.pjc_organization_id    = hru.organization_id(+)  
    and pda.pjc_project_id         = project.project_id(+)
    and pda.pjc_task_id            = pt.task_id(+)
    and pss.vendor_id              = psv.vendor_id
        /* Old logic- requisition number not getting populated 
    and pha.po_header_id           = rq.po_header_id(+)
    and pla.req_bu_id              = rq.req_bu_id(+)
    and pla.po_line_id             = rq.po_line_id(+) */
    and pha.type_lookup_code       = 'STANDARD'
    and pha.document_status not in ('INCOMPLETE','CANCELED','REJECTED')
    and pla.line_status not in ('INCOMPLETE','CANCELLED')

I expect the ability to retrieve all items numbers afiliated with the purchase order, with minimal changes.