query help

Is it not because you are inserting the extra join before the ON clause that defines the join between po and ap?

Does:

SELECT nvl(po.person_id,ap.employee_id) person_id,
              po.cost_centre po_cost_centre,
              ap.cost_center ap_cost_centre, ap.signing_limit ap_signing_limit
         FROM vzn_po_subordinate_cst_ctr_gtt po FULL OUTER JOIN
              ap_web_signing_limits ap
              ON (po.person_id = ap.employee_id AND po.cost_centre = ap.cost_center)
              ,gl_sets_of_books sob
        WHERE nvl(ap.document_type,'APEXP') = 'APEXP'
        and po.SET_OF_BOOKS_ID = sob.SET_OF_BOOKS_ID

Work, or even

SELECT nvl(po.person_id,ap.employee_id) person_id,
              po.cost_centre po_cost_centre,
              ap.cost_center ap_cost_centre, ap.signing_limit ap_signing_limit
         FROM vzn_po_subordinate_cst_ctr_gtt po FULL OUTER JOIN
              , gl_sets_of_books sob ,
              ap_web_signing_limits ap
              ON (po.person_id = ap.employee_id AND po.cost_centre = ap.cost_center)
        WHERE nvl(ap.document_type,'APEXP') = 'APEXP'
        and po.SET_OF_BOOKS_ID = sob.SET_OF_BOOKS_ID

Ignore my second example, its complete rubbish, I had not spotted the outer join keyword, try this instead if the first example does not work

SELECT nvl(po.person_id,ap.employee_id) person_id,
              po.cost_centre po_cost_centre,
              ap.cost_center ap_cost_centre, ap.signing_limit ap_signing_limit
         FROM vzn_po_subordinate_cst_ctr_gtt po FULL OUTER JOIN
              ap_web_signing_limits ap ON (po.person_id = ap.employee_id AND po.cost_centre = ap.cost_center)
              LEFT JOIN gl_sets_of_books sob ON po.SET_OF_BOOKS_ID = sob.SET_OF_BOOKS_ID
        WHERE nvl(ap.document_type,'APEXP') = 'APEXP'

Full outer join ... ON is one complete syntax, by putting comma, you are breaking the syntax, now oracle is not able to figure out that where is its joining condition, thats why ur receiving an error.

right, that's what I meant by "combining" syntaxes.

just one thing more can i replace
nvl with sumthing else in oracle
NVL(ap.document_type, 'APEXP') = 'APEXP'

NVL is specifically for null value, but you can use decode(ap.document_type, first_value,'1', second_value,'1', third_value,'1','APEXP')='APEXP'