Problem with GROUP BY/HAVING in Sybase -- Oracle Migration

Question

Having problem converting SQL from Sybase to Oracle. The SQL statement in Sybase does
not work in Oracle (ORA-979). When make changes the Oracle result is incorrect. Oracle Version
is 8.1.6.3.
#1 Sybase
SELECT
BIRTHDTE, FSCDTE, MINCDE, RETDISDTE, RETOPTDTE, RETPLAN,
RMPAYSTAT, RSCDTE, SEX, SSN, VETPREF, VIETVETIND, MAX(OBS_DATE)
FROM wims3.adm_new_stscrnt ans, wims3.tt_t_ssn2 t
WHERE (t.newssn = ans.SSN)
GROUP BY
SSN
HAVING obs_date = MAX(obs_date);
Result = 19,567 (Correct)
#2 Oracle (8.1.6.3)
SELECT
BIRTHDTE, FSCDTE, MINCDE, RETDISDTE, RETOPTDTE, RETPLAN,
RMPAYSTAT, RSCDTE, SEX, SSN, VETPREF, VIETVETIND, MAX(OBS_DATE)
FROM wims3.adm_new_stscrnt ans, wims3.tt_t_ssn2 t
WHERE (t.newssn = ans.SSN)
GROUP BY
SSN
HAVING obs_date = MAX(obs_date);
Result = ORA-00979 not a Group By expression
#3 Oracle
SELECT
BIRTHDTE, FSCDTE, MINCDE, RETDISDTE, RETOPTDTE, RETPLAN,
RMPAYSTAT, RSCDTE, SEX, SSN, VETPREF, VIETVETIND, MAX(OBS_DATE)
FROM wims3.adm_new_stscrnt ans, wims3.tt_t_ssn2 t
WHERE (t.newssn = ans.SSN)
GROUP BY
OBS_DATE, BIRTHDTE, FSCDTE, MINCDE, RETDISDTE, RETOPTDTE, RETPLAN,
RMPAYSTAT, RSCDTE, SEX, SSN, VETPREF, VIETVETIND
HAVING obs_date = MAX(obs_date);
Result = 19,598 (incorrect count)
Possible suggestions?

Answer 1

SELECT
BIRTHDTE, FSCDTE, MINCDE, RETDISDTE, RETOPTDTE, RETPLAN,
RMPAYSTAT, RSCDTE, SEX, SSN, VETPREF, VIETVETIND, OBS_DATE
FROM wims3.adm_new_stscrnt ans, wims3.tt_t_ssn2 t
WHERE (t.newssn = ans.SSN)
AND obs_date = (select MAX(obs_date) from wims3.adm_new_stscrnt ans2, wims3.tt_t_ssn2 t2 where ...)
... should be the primary key-where