Question

I’m having some trouble with finalizing my MySQL query. Imagine that i combine the tables tickets, invoice line and users. Each ticket has one or more invoice lines and each user can have one or more tickets.

I’m trying to create a query which I can use to create the name badges. For this badge I need to know the name and the items the person registered for.

I created the following MySQL query

SELECT u.firstName, u.lastName, il.invID, il.name, 
CASE 
  WHEN il.name = 'Conference Dinner' 
  THEN 'Diner'
END AS 'conference_dinner',

CASE 
  WHEN il.name = 'Regular Conference Fee' THEN 'Conference'
  WHEN il.name = 'Conference Fee for Phd. Students'  THEN 'Conference'
END AS 'conference',

CASE
  WHEN il.name = 'Pre-Conference Fee' THEN 'Pre-Conference'
END AS 'pre_conference',

FROM invoice_line il, events_tickets et, users u
WHERE il.invID = et.invID
AND et.userID = u.ID

The query is almost fine, however it produces for a user with three invoice lines (= three items) three lines in the table, and I’d like these three lines to merge to one line. A Group By on the users ID will result in one line, but than the case created columns won’t be merged into the one line, only one of them is displayed.

I might have used the wrong way of doing this. But It should result in an table like this:

firstName | lastName | conference_diner | conference | pre_conference |
----------------------------------------------------------------------
John      | Doe      | Diner            | Conference |                |
Jane      | Norman   |                  | Conference | Pre-Conference | 
Martijn   | Thomas   |                  | Conference |                |
Pete      | Johns    |                  |            | Pre-Conference |

Currently this results in:

firstName | lastName | conference_diner | conference | pre_conference |
----------------------------------------------------------------------
John      | Doe      | Diner            |            |                |
John      | Doe      |                  | Conference |                |
Jane      | Norman   |                  | Conference |                |
Jane      | Norman   |                  |            | Pre-Conference |  
Martijn   | Thomas   |                  | Conference |                |
Pete      | Johns    |                  |            | Pre-Conference |

Thanks in advance

Answer 1

You might try something along these lines:

SELECT u.firstName, u.lastName, il.invID, il.name,
IF(SUM(il.name = 'Conference Dinner') > 0,
    'Diner', '') AS 'conference_dinner',
IF(SUM(il.name IN ('Regular Conference Fee',
                   'Conference Fee for Phd. Students')) > 0,
    'Conference', '') AS 'conference',
IF(SUM(il.name = 'Pre-Conference Fee') > 0,
    'Pre-Conference', '') AS 'pre_conference'
FROM invoice_line il, events_tickets et, users u
WHERE il.invID = et.invID
AND et.userID = u.ID
GROUP BY u.ID

A comparison in mysql results in a number, 0 for false and 1 for true. So you can simply sum those up to count the lines matching a given expression. In other words, all the lines from the join which belong to a single user are grouped, those matching a certain property are counted, and these counts will decide what to print as the aggregate value for that user in the resulting table.