Re: DISTINCT ... ORDER BY

Bruno Wolff III wrote:

> You can use a distinct on with order by clause as a subselect to eliminate
> the records you don't want. Then you can extract col3 from this subselect
> and order by the original sort order (instead of col3 first) to get the
> desired records in the desired order.
>
> It would look something like:
> select col3 from
>   (select distinct on (col3) * from tablename order by col1, col2, col3, col4)
>   order by col1, col2, col3, col4;

Unfortunately I need to ORDER BY expression, not plain columns.

SELECT DISTINCT ON (col_1) col_1 FROM tab ORDER BY col_2='foo' DESC, col_1='bar' DESC, col_3='blah' DESC;

fails with:

ERROR:  SELECT DISTINCT ON expressions must match initial ORDER BY expressions

:(

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