Oracle query by column1 where column2 is the same - oracle

Question

I have a table like this in Oracle 9i DB:
+------+------+
| Col1 | Col2 |
+------+------+
| 1 | a |
| 2 | a |
| 3 | a |
| 4 | b |
| 5 | b |
+------+------+
Col1 is the primary key, Col2 is indexed.
I input col1 as condition for my query and I want to get col1 where col2 is the same as my input.
For example I query for 1 and the result should be 1,2,3.
I know I can use self join for this, I would like to know if there is a better way to do this.

Answer 1

I'd call this a semi-join: does it satisfy your 'no self joins' requirement?:
SELECT *
FROM YourTable
WHERE Col2 IN ( SELECT t2.Col2
FROM YourTable t2
WHERE t2.Col1 = 1 );
I'd be inclined to avoid the t2 range variable like this:
WITH YourTableSearched
AS ( SELECT Col2
FROM YourTable
WHERE Col1 = 1 )
SELECT *
FROM YourTable
WHERE Col2 IN ( SELECT Col2
FROM YourTableSearched );
but TNH I would probably do this:
WITH YourTableSearched
AS ( SELECT Col2
FROM YourTable
WHERE Col1 = 1 )
SELECT *
FROM YourTable
NATURAL JOIN YourTableSearched;

Answer 2

It's possible. Whether it's better (i.e. more performant) than using a self-join, particularly if there is an index on col1, col2, is anyone's guess.
Assuming col1 is unique, you could do:
SELECT col1
FROM (SELECT col1,
col2,
MAX(CASE WHEN col1 = :p_col1_value THEN col2 END) OVER () col2_comparison
FROM your_table)
WHERE col2 = col2_comparison;
And with :p_col1_value = 1:
COL1
----------
1
2
3
And with :p_col1_value = 5:
COL1
----------
4
5