I have a query that is a union with two selects. When the top select is run seperately it returns 6 rows. When the bottom query is run it returns 0 rows. When the whole query is run it returns 4 rows. Why is this odd behavior happening?

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elect
sat.service_account_number,
nvl(sat.service_id, -99) ,
nvl(sat.service_type ,'NULL'),
nvl(s.service_type, 'NULL') service_service_type,
nvl(rc.rate_category ,'NULL'),
nvl(att.transaction_type, 'NULL') at_trans_type,
sat.transaction_type sat_trans_type,
nvl(sat.read_class, 'NULL'),
trunc(sat.trans_date) trans_date,
sum(sat.trans_amount) trans_amount,
sum(sat.allocation_unit) sat_allocation_unit,
max(att.gst_exempt) at_gst_exempt,
max(att.pst_exempt) at_pst_exempt,
max(att.payment_method_type) at_payment_method_type,
max(att.accounting_period) at_accounting_period,
max(sat.gl_number) sat_gl_number,
max(sat.offset_gl_number) sat_offset_gl_number,
nvl(sat.billing_charge_type, 'NULL'),
max(sat.wahsp) sat_wahsp,
nvl(sat.rate_period_id ,-99),
nvl(rc.rate_class_billing_type, 'Z'),
att.account_number,
nvl(rp.tlf_indicator, 'Z'),
nvl(rc.rate_class_id,-99),
ac.account_type
from cis.account_transaction@standby.world att,
cis.service_acct_transaction@standby.world sat,
cis.service@standby.world s,
cis.read_class@standby.world r,
cis.rate_period@standby.world rp,
cis.rate_class@standby.world rc,
cis.service_account@standby.world sa,
cis.account@standby.world ac
/*where sat.service_acct_trans_id >= n_start_service_acct_tran_id
and sat.service_acct_trans_id <= n_end_service_acct_tran_id*/
where sat.service_id = 63923
and trunc(sat.trans_date) = '26-nov-2001'
and sat.transaction_type = 'WMRENT'
and att.account_transaction_id = sat.account_transaction_id (+)
and sat.service_id = s.service_id (+)
and sat.read_class = r.read_class (+)
and sat.rate_period_id = rp.rate_period_id (+)
and rp.rate_class_id = rc.rate_class_id (+)
and sat.service_account_number = sa.service_account_number
and sa.account_number = ac.account_number
group by att.account_number,ac.account_type, sat.service_account_number,
sat.service_id,sat.service_type ,
s.service_type,sat.billing_charge_type,
rc.rate_category,att.transaction_type,
sat.transaction_type,
sat.read_class,sat.rate_period_id,
rp.tlf_indicator,rc.rate_class_id,
rc.rate_class_billing_type,
att.trans_date,sat.trans_date
union -- Service Account Transaction records with a null Account Transaction Id
select sat.service_account_number,
nvl(sat.service_id, -99) ,
nvl(sat.service_type , 'NULL') sat_service_type,
nvl(s.service_type, 'NULL') service_service_type,
nvl(rc.rate_category ,'NULL'),
'NULL' at_trans_type,
sat.transaction_type sat_trans_type,
nvl(sat.read_class, 'NULL'),
trunc(sat.trans_date) trans_date,
sum(sat.trans_amount) trans_amount,
sum(sat.allocation_unit) sat_allocation_unit,
'N' at_gst_exempt,
'N' at_pst_exempt,
'NULL',
to_char(sysdate,'YYYYMM') at_accounting_period,
max(sat.gl_number) sat_gl_number,
max(sat.offset_gl_number) sat_offset_gl_number,
nvl(sat.billing_charge_type, 'NULL'),
max(sat.wahsp) sat_wahsp,
nvl(sat.rate_period_id ,-99),
nvl(rc.rate_class_billing_type, 'Z'),
sa.account_number,
nvl(rp.tlf_indicator, 'Z'),
nvl(rc.rate_class_id,-99),
ac.account_type
from cis.service_acct_transaction@standby.world sat,
cis.service@standby.world s,
cis.read_class@standby.world r,
cis.service_account@standby.world sa,
cis.account@standby.world ac,
cis.rate_period@standby.world rp,
cis.rate_class@standby.world rc
/* where sat.service_acct_trans_id >= n_start_service_acct_tran_id
and sat.service_acct_trans_id <= n_end_service_acct_tran_id
*/
where sat.service_id = 63923
and trunc(sat.trans_date) = '26-nov-2001'
and sat.transaction_type = 'WMRENT'
and sat.account_transaction_id is null
and sat.service_id = s.service_id (+)
and sat.read_class = r.read_class (+)
and sat.service_account_number = sa.service_account_number
and sa.account_number = ac.account_number
and sat.rate_period_id = rp.rate_period_id (+)
and rp.rate_class_id = rc.rate_class_id (+)
group by sa.account_number,
ac.account_type,
sat.service_account_number,
sat.service_id,
sat.service_type ,
sat.billing_charge_type,
rc.rate_category,
s.service_type,
sat.transaction_type,
sat.read_class,
sat.rate_period_id,
rp.tlf_indicator,
rc.rate_class_id,
rc.rate_class_billing_type,
sat.trans_date

it's early, very early... so forgive me if i lead you astray... but try a UNION ALL