在该查询中，我使用哪种JOIN类型显示NULL值？

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; I'm currently not using this at all) and Title .

---

What I'm trying to do, provided with a specific list of student IDs, is display a list of students' points AND three most expensive reward purchases.

The SQL statement I've written thus far is:

SELECT
  t.Recipient_ID AS  `ID` ,
  SUM( t.Points ) AS  `Points Earned`,
  SUBSTRING_INDEX( GROUP_CONCAT(DISTINCT r.Title SEPARATOR ', '),', ',3 ) as `Rewards`
FROM  `transactions` t
JOIN `purchases` p ON t.Recipient_ID = p.Student_ID
JOIN `rewards` r ON p.Reward_ID = r.Reward_ID
WHERE  T.`Recipient_ID` 
  IN ( 90128, 90163, 34403, 35501 )
GROUP BY  t.`Recipient_ID`

This works to an extent - two students are displayed, with their point totals and latest rewards.

However, their point totals are vastly wrong, and I believe students aren't displaying if they haven't made any purchases.

I would still like to display the students if they've made no purchases.

---

I suspect it's my JOINs which are incorrect, but I also suspect GROUP_CONCAT isn't powerful enough to list the three most EXPENSIVE rewards. Do I need to change my JOINs, or do I need to use some sort of subquery?

Thanks in advance,

Answer 1

I think you should look at using a LEFT JOIN (See Visual Explanation of Joins ):

SELECT
  t.Recipient_ID AS  `ID` ,
  SUM( t.Points ) AS  `Points Earned`,
  SUBSTRING_INDEX( GROUP_CONCAT(DISTINCT r.Title 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, 34403, 35501 )
GROUP BY  t.`Recipient_ID`

A LEFT JOIN will return all records from the transactions table even if there are no records in the purchases or rewards table.