How to select distinct field values using Solr?

I would like to do the equivalent of this SQL but with Solr as my data store.

SELECT
   DISTINCT txt
FROM
   my_table;

What syntax would force Solr to only give me distinct values?

http://localhost:8983/solr/select?q=txt:?????&fl=txt

EDIT: So faceted searching seems to fit, but as I investigated it, I realized I had only detailed half of the problem.

My SQL query should have read...

SELECT
   DISTINCT SUBSTR(txt,0,3)
FROM
   my_table;

Any possibility of this with Solr?