Oracle 11g Release 2

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
create table table2 (num1 varchar(20), num2 varchar(20))
insert into table2 values('a', 'aa1')
1 rows affected

insert into table2 values('a', 'aa2')
1 rows affected

insert into table2 values(' ', 'bb')
1 rows affected

insert into table2 values(' ', 'bb')
1 rows affected

insert into table2 values(' ', 'cc')
1 rows affected

create table table1 (num2 varchar(20), count int)
insert into table1 values('aa1', 10)
1 rows affected

insert into table1 values('aa2', 5)
1 rows affected

insert into table1 values('bb', 2)
1 rows affected

insert into table1 values('bb', 4)
1 rows affected

insert into table1 values('cc', 80)
1 rows affected

SELECT CASE WHEN t.num1 = ' ' THEN 
                 to_char(k.num2)
       ELSE 
                 t.num1
       END num1
       , MIN(k.count)  count_MIN
       , MAX(k.count)  count_MAX
FROM table1 k
LEFT JOIN table2 t ON k.num2 = t.num2
WHERE k.count > 1
GROUP BY CASE WHEN t.num1 = ' ' THEN TO_CHAR(k.num2)
              ELSE t.num1
         END;


NUM1
COUNT_MIN
COUNT_MAX


bb
2
4

a
5
10

cc
80
80


SELECT (CASE
WHEN t.num1 = ' ' THEN TO_CHAR(k.num2
ELSE t.num1
END) AS 'num1',
MIN(k.count) AS 'count_MIN',
MAX(k.count) AS 'count_MAX'
--
FROM table1 AS k
--
LEFT JOIN table2 AS t
ON k.num2 = t.num2
--
WHERE k.count > 1
--
GROUP BY (CASE
WHEN t.num1 = ' ' THEN TO_CHAR(k.num2
ELSE t.num1
END)
--
;
ORA-00907: missing right parenthesis