DISTINCT ... ORDER BY

Hi all,

I have built a query that mainly orders all columns.
It shouldn't matter how it orders but FYI:
I need to order by 1 given column and then by all others from 'left to right'.

No problem so far.

Then I want to return only 1 column (e.g. the 3rd) with duplicates removed
_AND_ still in the same order like above.

e.g.:

myCol
-----
a
a
c
b
b
c
b

should return:

myDistinctOrderedCol
--------------------
a
c
b

The problem is that DISTINCT complains about ORDER BY entries not being in the target list.
Any idea ?

Example:

initial query:
SELECT * FROM tab ORDER BY col_2='foo' DESC, col_1='bar' DESC, col_3='blah' DESC;

Then something like:
SELECT DISTINCT col3 FROM {initial query};

TIA
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