DINSTINCT without COUNT, in "selector" mode.
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Please don't fill out this field.
The problem with DISTINCT is that it is not a column modifier, it is the whole SELECT modifier. It is possible to execute e.g. SELECT DISTINCT CHAR_LENGTH(column). So there would have to be a new checkbox for DISTINCT which would complicate the user interface (it is not such common operation).
When I personally want to list distinct values then I select column together with COUNT(*) which would create GROUP BY column which is basically the same as DISTINCT.
To enable this feature, you can create your own customization: http://www.adminer.org/cs/extension/ (methods selectColumnsPrint and selectColumnsProcess).