Any idea for a SQL Query?

Question

I have a table with 2 columns. I want to select the following rows:
a) any of the 2 rows where Row 1: col1, col2 = Row2: col2, col1
b) all other rows where there is no equality constraint
Example:
Col1 Col2
ABC001 || BC987
BC987 || ABC001
CDE || PH
BC987 || XYZ
Result should be
a) any of the first 2 rows because col1 value in 1st row is same as col2 value in 2nd row; and col2 value in 1st row is same as col1 value in 2nd row.
b) 2nd and 3rd rows (because these are unique rows)
Data setup:
create table temptab (col1 varchar2(20), col2 varchar2(20))
insert into temptab values ('ABC001', 'BC987')
insert into temptab values ('BC987', 'ABC001')
insert into temptab values ('CDE', 'PH')
insert into temptab values ('BC987', 'XYZ')
Thanks

Answer 1

Hi,
The ORDER BY clause of the ROW_NUMBER fucntion determines which row will be called #1, whcih will be #2, and so on.
I used
...                         ORDER BY        col1more or less arbitrarily. You can use anything you like, such as:
...                         ORDER BY        CASE
                                   WHEN  typ = 'Adaptive'
                                   THEN  1
                                         ENDThis means that, if any rows with typ='Adaptive' are in the group, then one of them will be labeled #1. Which one? Who knows (and who cares)? If you actually do care, then you can add another expression to the ORDER BY clause; just make sure you add it after the CASE expression, so that the new rule comes into play only after the 'Adaptive'/Other test has been done.
If 'Adaptive' is the first thing (in VARCHAR2 sort order) that can be in the typ column (and NULL comes after everything else), then you could simply say:
...                         ORDER BY        typPersonally, I don't hink that's a good idea. Suppose that, two years form mow, they make 'Aardvarkian' a valid typ. What are the chances that the person doing your job (even if that's you) is going to remember to change queries like this?