try with UNION
SELECT COMPNO FROM REF10 WHERE SR_NO='RDDFT000108';
UNION ALL;
SELECT COMPNO FROM REF11 WHERE SR_NO='RDDFT000108';
UNION;
SELECT COMPNO FROM REF12 WHERE SR_NO='RDDFT000108';
UNION;
SELECT COMPNO FROM REF08 WHERE SR_NO='RDDFT000108';
UNION;
SELECT COMPNO FROM REF09 WHERE SR_NO='RDDFT000108';