Answer the following question with the SQL query that I need for the answer.

1. Using only the documents table, how many different participants (usernames) did participate in the experiment? (remember; the query should return a single(!) number)

A single number!!!!!

a 101 yunistate.edu/ba371/reitsma/assignment assignes o participants table: -- - | Field Type --- username varchar(255) | user type varchar(10) 1 years int | low grade int high grade int on line varchar(10) on_line_sources | varchar(255) | location varchar(5) exp_condition int -- ---- --- This table contains the information for each of the experiment's participants - username: id of the participant. user_type: was the participant a student or a licensed teacher. years: years of teaching experience. - low_grade: lowest grade the participant teaches. - high grade: highest grade the participant teaches. - on_line: does the participant use on-line lesson materials? - on_line_sources: if so, which ones? location: Subject's US state. exp_condition: 1=list, 2=map. tasks table: | Field apce e CUTIE tasks table: Field - --- ---- --- - + username task confidence sim helpd | varchar(255) | varchar(5) int int +-.. This table lists the various tasks participants conducted during the experiment: - username: id of the participant. taskid of the task the participant completed. confidence: confidence that the participant had in having done the task well (1-6). sim_helpd: participant's opinion that the map or list (depending on which one they documents table: Field Type username yarchar255 JUULIUL UL WILLY USEU Was lepi documents table: Field Type | username I varchar(255) | task i varchar(5) doc_type varchar(10) I used tool int relevant int motivational | concepts background | grade_level I hands on | attachments . ... SEE Participants were asked to search for documents on the web site as part of their tasks. Each row in the table - username: id of the participant task: id of the task the participant completed doc_type: the type of the found document. - used_tool attachments: opinion about the usefulness of the found document (1-6). experiment_data table: Field Used TOOL LUCmen. Upun douu ule useless U U UUD o experiment_data table: Field Type int(11) host_ip varchar (20) referer varchar(255) file varchar(255) querystring varchar(255) timestamp datetime username varchar(255) source doc id int(11) dest_doc_id int(11) exp_condition | int(11) map action int(11) All of the participants' actions associated with the 'list' and 'map' interfaces - id: record id host_ip: IP of the machine from which the request came. referer: referring web page. file: requested web page. querystring any parameters passed in the request. timestamp: time of the request (one-second accuracy). username: id of the participant. source_doc_id: the id of the document from which the request/action i dest_doc_id: the id of the document resulting from the action. - exp_condition: 1=list, 2=map. map_action: type of map action executed