Incorrect results from three table join - mysql

Question

I have these three tables in my database:
tblCustomer (id,name,address)
tblLoan (id,customerId,LoanAmount,date)
tblPayment (id,customerId,ReceivedAmount,date)
I want to find the total loanAmount for a customer and how much they have paid.
I wrote this query:
SELECT c.fname, SUM(l.amount), SUM(p.amount)
FROM tblCustomer c
JOIN tblLoan l ON (l.customerId = c.id)
JOIN tblPayment p ON (p.customerId = c.id)
WHERE c.id = 3;
It returns results but they are incorrect.

Answer 1

First, as others have mentioned, your syntax is likely incorrect because you do not have matching column names, but you said you had incorrect results, so I would assume that's not your problem as you were able to run your query..
The problem that I think you are most likely having is that by joining the two tables together like that, rows appear twice for each customer. Am I correct in assuming that your 'incorrect' results are double what you would expect? Let me illustrate for those who don't understand. Consider this data set, with shortened column values:
tblCustomer:
| id | name |
+----+------+
| 1 | Adam |
| 2 | John |
| 3 | Jane |
tblLoan, and for simplicity we'll say the payment table looks the same:
| customerID | loanAmount |
+------------+------------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
| 3 | 300 |
| 2 | 200 |
If I perform the following query (without summing values, just getting the values I want:
SELECT c.id, c.name, l.loanAmount, p.receivedAmount
FROM tblCustomer c
JOIN tblLoan l ON l.customerid = c.id
JOIN tblPayment p ON p.customerid = c.id
WHERE c.id = 3;
It returns this result set:
| id | name | loanAmount | receivedAmount |
+----+------+------------+----------------+
| 3 | Jane | 100 | 100 |
| 3 | Jane | 100 | 300 |
| 3 | Jane | 300 | 100 |
| 3 | Jane | 300 | 300 |
So notice that because we're joining two tables based on a relationship to a third table, were actually creating a cartesian product which is causing the problem. So, what I recommend you do is use subqueries for these two tables. One subquery will pull the loan values, one the payment values, and you can join those together on the id value.
It will look like this:
SELECT t.id, t.totalLoan, w.totalReceived
FROM(SELECT c.id, SUM(l.loanAmount) AS totalLoan
FROM tblCustomer c
JOIN tblLoan l ON l.customerid = c.id
WHERE c.id = 3) t
JOIN(SELECT c.id, SUM(p.receivedAmount) AS totalReceived
FROM tblCustomer c
JOIN tblPayment p ON p.customerid = c.id
WHERE c.id = 3) w
ON t.id = w.id;
And this should give you the values you want. Here is what I tested on SQL Fiddle.

Answer 2

FYI, YOUR COLUMN NAMES ARE WRONG!!!
There is no such column named fname in table tblCustomer
There is no such column named amount in table tblLoan
There is no such column named amount in table tblPayment
You won't get the right result if you don't have the appropriate column names. Even when using aliases, your column name should be EXACTLY THE SAME as in your database table. That's because, you are aliasing TABLES in JOIN queries, not COLUMNS.
So, re-write your query in the following way:
SELECT c.name, SUM(l.LoanAmount), SUM(p.ReceivedAmount)
FROM tblCustomer c
JOIN tblLoan l ON l.customerId = c.id
JOIN tblPayment p ON p.customerId = c.id
WHERE c.id = 3
Note that there's no need to get brackets around the ON clause in JOIN.