Combining multiple queries into a single query
Yes, you can update the group in one go using an UPDATE statement like this:

UPDATE
  exp_channel_data
SET
  field_id_285 =
    CONCAT_WS(
      '\n',
      CASE WHEN field_id_151 &lt;&gt; '' THEN 'Thursday' END,
      CASE WHEN field_id_152 &lt;&gt; '' THEN 'Friday'   END,
      CASE WHEN field_id_153 &lt;&gt; '' THEN 'Saturday' END,
      CASE WHEN field_id_154 &lt;&gt; '' THEN 'Sunday'   END
    )
WHERE
  channel_id = 14
  AND (field_id_151 &lt;&gt; ''
    OR field_id_152 &lt;&gt; ''
    OR field_id_153 &lt;&gt; ''
    OR field_id_154 &lt;&gt; ''
  )
;


Each of the CASE expressions produces either a week day name or null. The CONCAT_WS function concatenates the names using \n as the delimiter, omitting the nulls, if any.

However, even though you can do this with a single statement, the fact remains that this is a terrible design. Without more information it is hard to suggest a proper alternative but it seems likely that a junction table could be used here instead.