SELECT field1,field2,field3,field4,field5,field6,field7,field8,
min(START_DATE) as START_DATE ,max(END_DATE) as END_DATE
FROM (
SELECT field1,field2,field3,field4,field5,field6,field7,field8,START_DATE,END_DATE FROM RG
UNION
SELECT field1,field2,field3,field4,field5,field6,field7,field8,START_DATE,END_DATE from RA
UNION
SELECT field1,field2,field3,field4,field5,field6,field7,field8,START_DATE,END_DATE FROM SR
UNION
SELECT field1,field2,field3,field4,field5,field6,field7,field8,START_DATE,END_DATE
FROM GT) t
GROUP BY field1,field2,field3,field4,field5,field6,field7,field8;