Selecting and joining rows from multiple tables based on MAX(Column Name)

Question

Overall, my goal is to obtain a customer's email, code, and most recent rewards balance from three tables.

The three tables are: Customer, CustomerCode, and Rewards

The tables look roughly like the following...

Customer

id   email           lastcode
----|---------------|-----------
000 |test@test.com  | 1234test
001 |test1@test.com | 5678test
002 |test2@test.com | test1234
003 |test3@test.com | test5678

CustomerCode

id   code      customer
----|---------|---------
100 |1234test | 000
101 |5678test | 001
102 |test1234 | 002
103 |test5678 | 003

Rewards

customercode  logdate      balance
-------------|------------|--------
100          | 01/01/2016 | 1200
101          | 04/05/2016 | 40
102          | 06/22/2016 | 130
102          | 10/14/2016 | 220
103          | 12/03/2016 | 500
103          | 01/18/2017 | 750

I'm trying to gather information from all the tables linking back to the customer. I'm currently using the following SQL query but am running into some issues.

SELECT Customer.email, Customer.lastcode, CustomerCode.id, Rewards.balance, MAX(Rewards.logdate)
FROM Customer
JOIN CustomerCode ON Customer.lastcode=CustomerCode.code
JOIN Rewards ON CustomerCode.id=Rewards.CustomerCode
GROUP BY Customer.Email, Customer.LastCode, CustomerCode.id, Rewards.Balance

Results

As you can see, I'm getting multiple results for the same customer, but I only want to get the most recent rewards balance for each customer.

email           lastcode    id    balance    logdate
---------------|-----------|-----|----------|-----------
test@test.com  | 1234test  | 100 | 1200     | 01/01/2016
test1@test.com | 5678test  | 101 | 40       | 04/05/2016
test2@test.com | test1234  | 102 | 130      | 06/22/2016
test2@test.com | test1234  | 102 | 220      | 10/14/2016
test3@test.com | test5678  | 103 | 500      | 12/03/2016
test3@test.com | test5678  | 103 | 750      | 01/18/2017

Is there any way I can eliminate those duplicate records and only display the most recent rewards balance?

Answer 1

You can use a correlated subquery or aggregation for this:

SELECT c.email, c.lastcode, cc.id, r.balance, r.logdate
FROM Customer c JOIN
     CustomerCode cc
     ON c.lastcode = cc.code JOIN
     Rewards r
     ON cc.id = r.CustomerCode JOIN
     (SELECT r.CustomerCode, MAX(r.logdate) as max_logdate
      FROM Rewards r
      GROUP BY r.CustomerCode
     ) rr
     ON rr.CustomerCode = r.CustomerCode AND rr.max_logdate = r.logdate;

Answer 2

sth like this?

SELECT
  costumer.email,
  costumer.lastcode,
  reward.costumercode,
  reward.balance
FROM (SELECT DISTINCT
        rewards.costumercode,
        MAX(rewards.balance) as balance
      FROM rewards
      GROUP BY 1) AS reward
JOIN costumercode ON costumercode.id = reward.costumercode
JOIN costumer ON costumer.id = costumercode.costumer

It is more optimized than Gordon's anwser, and more clean IMHO.