Oracle 11g Release 2

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with HeaderTable (MAIN_NO, MAIN_DESCRIPTION, MAIN_CUST, MAIN_NAME) as (
            select 1, 'Apple', 'Student', 'John' from dual
  union all select 2, 'Pear', 'Child', 'Peter' from dual
  union all select 3, 'Orange', 'Alien', 'Matt' from dual
),
DetailTable (MAIN_NO, PERF, EVENT, INFO, PREV_VALUE, NEW_VALUE, REV, STATE, MODIFIED, NOTE) as (
            select 1, 'ABC', 'YES', 'RED', 12, 30, 0, 'Con', to_date('01/02/2018', 'DD/MM/YYYY'), 'FALSE' from dual
  union all select 1, 'DEF', 'YES', 'BLUE', 18, 34, 0, 'Con', to_date('01/04/2018', 'DD/MM/YYYY'), 'FALSE' from dual
  union all select 2, 'GHI', 'NO', 'BLUE', 14, 38, 0, 'UUU', to_date('01/07/2018', 'DD/MM/YYYY'), 'TRUE' from dual
  union all select 2, 'JKL', 'NO', 'RED', 11, 39, 0, 'Con', to_date('01/03/2018', 'DD/MM/YYYY'), 'FALSE' from dual
  union all select 3, 'MNO', 'NO', 'GREEN', 19, 32, 0, 'UUU', to_date('01/01/2018', 'DD/MM/YYYY'), 'TRUE' from dual
  union all select 3, 'PQR', 'YES', 'WHITE', 10, 33, 0, 'UUU', to_date('01/08/2018', 'DD/MM/YYYY'), 'TRUE' from dual
)
SELECT MAIN_NO, MAIN_DESCRIPTION, MAIN_CUST, MAIN_NAME,
  PREV_VALUE, NEW_VALUE, REV, STATE, MODIFIED, NOTE
FROM (
    SELECT MAIN_NO, MAIN_DESCRIPTION, MAIN_CUST, MAIN_NAME,
        cast(null as number) as prev_value,
        cast(null as number) as new_value,
        cast(null as number) rev,
        cast(null as varchar2(3)) as state,
        cast(null as date) as modified,
        cast(null as varchar2(5)) as note,
        0 rnk
    FROM HeaderTable
    UNION ALL
    SELECT hd.MAIN_NO, hd.PERF, hd.EVENT, hd.INFO, hd.PREV_VALUE, hd.NEW_VALUE,
        hd.REV, hd.STATE, hd.MODIFIED, hd.NOTE,
        ROW_NUMBER() over (PARTITION BY hd.MAIN_NO ORDER BY hd.MAIN_NO) as rnk
    FROM HeaderTable h 
    INNER JOIN DetailTable hd on h.MAIN_NO= hd.MAIN_NO
)
ORDER BY main_no, rnk

MAIN_NO	MAIN_DESCRIPTION	MAIN_CUST	MAIN_NAME	PREV_VALUE	NEW_VALUE	REV	STATE	MODIFIED	NOTE
1	Apple	Student	John	null	null	null	null	null	null
1	ABC	YES	RED	12	30	0	Con	01-FEB-18	FALSE
1	DEF	YES	BLUE	18	34	0	Con	01-APR-18	FALSE
2	Pear	Child	Peter	null	null	null	null	null	null
2	GHI	NO	BLUE	14	38	0	UUU	01-JUL-18	TRUE
2	JKL	NO	RED	11	39	0	Con	01-MAR-18	FALSE
3	Orange	Alien	Matt	null	null	null	null	null	null
3	MNO	NO	GREEN	19	32	0	UUU	01-JAN-18	TRUE
3	PQR	YES	WHITE	10	33	0	UUU	01-AUG-18	TRUE