To find unique date... Good question

Question

Consider the following tables in your schema.
1. TOPTEN_SHIPS
2. TOPTEN_BATTLES
A typical SHIPS record would be:
(1,'New Jersey', 1943, 'USA', 9, 16, 46000)
Which means that the battleship New Jersey with id 1 was launched in 1943; it belonged to the USA, carried 9 guns of size 16-inch (bore, or inside diameter of the barrel), and weighted (displaced, in nautical terms) 46,000 tons.
A typical record for BATTLES is:
(1, 'North Atlantic', 1,'sunk')
That is, ship with id 1 (i.e. New Jersey) was ‘sunk’ in the battle (with id 1) of the North Atlantic.
Write the following queries:
(a) Grade: Medium
Recommended Time to be spent on the question: 20 min
List all the pairs of countries that fought each other in battles. List each pair only once, and list them with the country that comes first in alphabetical order first. Let’s say the country names are C1 and C2. C1 should appear first over C2 based on alphabetical order. Additionally C1 should only start with alphabet A.
The output should be ordered by country1, country2.
TOPTEN_SHIPS          
TS_SHIP_ID     NOT NULL     NUMBER(10)
TS_SHIP_NAME          VARCHAR2(100)
TS_YEAR_LAUNCHED          DATE
TS_COUNTRY          VARCHAR2(100)
TS_NUM_GUNS          NUMBER(10)
TS_GUN_SIZE          NUMBER(10,2)
TS_DISPLACEMENT          NUMBER(10)
TOPTEN_BATTLES          
TB_BATTLE_ID     NOT NULL     NUMBER(10)
TB_BATTLE_NAME          VARCHAR2(100)
TB_SHIP_ID     NOT NULL     NUMBER(10)
TB_RESULT          VARCHAR2(50)

Answer 1

if you want a generic method you can do something like this...
WITH t AS
  ( SELECT 'A' c1,'B' c2,'C' c3 FROM dual
UNION ALL
   SELECT 'A' c1,'B' c2,'D' c3 FROM dual
UNION ALL
   SELECT 'B' c1,'C' c2,'A' c3 FROM dual
UNION ALL
   SELECT 'C' c1,'A' c2,'B' c3 FROM dual
UNION ALL
   SELECT 'D' c1,'B' c2,'A' c3 FROM dual
SELECT DISTINCT ltrim(sys_connect_by_path(cvalue,','),',') finval
   FROM
  (SELECT c1           ,
    c2                 ,
    c3                 ,
    column_value cvalue,
    row_number() over (partition BY c1,c2,c3 order by column_value ) rnum
     FROM t,
    TABLE(sys.odcivarchar2list(c1,c2,c3))
  WHERE CONNECT_BY_ISLEAF =1
  CONNECT BY prior rnum   =rnum-1
AND prior c1              =c1
AND prior c2              =c2
AND prior c3              =c3
  START WITH rnum         =1But still if you want unique out of 4 columns then you need to change the above one accordingly.
Ravi Kumar