如何在 SQL 的单列中显示多列？

Question

I have a table attendee, when joining table attendee with table Session, attendee can attend many sessions. So when showing the attendees and what sessions they attended, I am getting multiple rows of the same Attendee, example:

John Doe-BMW Conference
John Doe-Blockchain conference
Jane Doe-blabla
John Doe- Mercedess

Is there a way in sql to display

John Doe- BMW Conference,Mercedes,Block chain?

SELECT Distinct
  attendee.Id, attendee.Firstname, attendee.PhoneNumber,
  attendee.Email,attendee.Town, attendee.BloodType, session.Id ,
  session.LocationId , session.Name as SessionName , location.Id ,
  location.Name as Location_Name , sessionattended.SessionAttendedId,
  sessionattended.SessionId, sessionattended.AttendeeId,
  attendee.Lastname
FROM `session`, `attendee`, `sessionattended`, `location`
WHERE attendee.Id = sessionattended.AttendeeId 
  and session.Id = sessionattended.SessionId 
  and session.LocationId = location.Id;

Answer 1

use GROUP_CONCAT MySQL (available since a long time)

or STRING_AGG MSSQL (available in version 2017 and later)

for a comparison of both, see: https://database.guide/mysql-group_concat-vs-t-sql-string_agg/

Google will find info on how to do this in other DBMS's, or even how to do it 'the old way', like here: GROUP BY to combine/concat a column

Answer 2

Try this. Basically the idea is to use FOR XML PATH for this type of result.

SELECT Distinct
       a.Id, a.Firstname, a.PhoneNumber, a.Email,a.Town, a.BloodType, 
       all_conferences = STUFF((SELECT ',' + CAST(s1.Name as Varchar) 
                                FROM sessionattended sa1  
                                JOIN Session s1 ON sa1.SessionId = s1.Id
                                WHERE sa1.AttendeeId = sa.AttendeeId FOR XML PATH('') ), 1, 1, '')

FROM `session` s
JOIN `sessionattended` sa ON sa.SessionId = s.Id
JOIN `attendee` a ON a.Id = sa.AttendeeId 
JOIN `location` l ON s.LocationId = l.Id