Question

I’m trying to join THREE tables.

The first table is transactions which contains details of “points” that students have earned. The relevant field here is Recipient_ID (the student receiving points).

The second table is purchases which contains details of “rewards” that students have purchased. The relevant fields here are Student_ID and Reward_ID.

The final table is rewards which details the rewards purchased. The relevant fields here are Reward_ID, Cost_to_User (the price the students pay for each reward which is being used to order the GROUP_CONCAT function) and Title.

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The query I’m using is as follows:

SELECT
  t.Recipient_ID AS `ID` ,
  SUM( t.Points ) AS `Points Earned`,
  SUBSTRING_INDEX( GROUP_CONCAT(DISTINCT r.Title ORDER BY r.Cost_to_User DESC SEPARATOR ', '),', ',3 ) AS `Rewards`
FROM  `transactions` t
LEFT JOIN `purchases` p 
   ON t.Recipient_ID = p.Student_ID
LEFT JOIN `rewards` r 
  ON p.Reward_ID = r.Reward_ID
WHERE t.`Recipient_ID` 
  IN ( 90128, 90163, 33888, 34240, 137674 )
GROUP BY  t.`Recipient_ID`

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For some reason, the Points Earned output is vastly wrong. If I remove the LEFT JOIN on purchases, the Points Earned output is correct.

What is it about that join that’s sending my SUM command crazy?

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EDIT: Please note that I require the LEFT JOIN commands as I’d like to display all students, including those that haven’t made any purchases. Further details found in this post.

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Thanks in advance,

Answer 1

The problem is that you are getting multiple records from the transaction table with the join. You can use a subquery on the transaction table and it should fix this issue:

SELECT t.`ID` ,
  t.`Points Earned`,
  SUBSTRING_INDEX( GROUP_CONCAT(DISTINCT r.Title ORDER BY r.Cost_to_User DESC SEPARATOR ', '),', ',3 ) AS `Rewards`
FROM
(
  select t.Recipient_ID AS `ID`, sum(t.Points) AS `Points Earned`
  from `transactions` t
  group by t.Recipient_ID
) t
LEFT JOIN `purchases` p 
   ON t.`ID` = p.Student_ID
LEFT JOIN `rewards` r 
  ON p.Reward_ID = r.Reward_ID
WHERE t.`ID` 
  IN ( 90128, 90163, 33888, 34240, 137674 )
GROUP BY  t.`ID`

See SQL Fiddle with Demo

The strange total of the points is coming from the join of the tables. Since you are joining the tables on only the Recipient_ID = Student_ID if there are records in both tables you are getting a cartesian number of results. So if you have 3 records for a student in the transactions table and 3 records in the purchases table the result will show 9 records for that student which alters the total.

See SQL Fiddle with Demo