Query on MYSQL JOINS(2 tables) - mysql

Question

I am new to Join tables.
I have a table 'Merchants' like the following
Merchant_Number Merchant_Name
1 ABCD
2 DEFG
I have a table 'Transactions' like the following
Merchant_Number Merchant_Name Transaction_Amount
1 ABCD 100
3 XYZ 50
1 ABCD 50
4 nnn 200
I want to join the above 2 tables such that the result is like the following
Merchant_Number Merchant_Name Transaction Amount No. Of Transactions
1 ABCD 150 2
2 DEFG 0 0
3 XYZ 50 1
4 nnn 200 1
Transaction Amount is the sum of the amount for the same merchant
No. Of transactions is the number of times the merchant used his transaction
Merchant ABCD has 2 transactions and his total transaction amount is 150
I have a query
SELECT Merchant_Name, sum(transaction_amount) as 'Transaction Amount',
count(*) as 'No. Of Transactions'
FROM Transactions
LEFT JOIN Merchants using (MERCHANT_NUMBER)
group by Merchant_Name
But this gives me only the merchants in transaction table i.e,
Merchant_Number Merchant_Name Transaction Amount No. Of Transactions
1 ABCD 150 2
3 XYZ 50 1
4 nnn 200 1
How do I query such that I have my result as explained above?
Any Help would be highly appreciated!

Answer 1

You want the join the other way around, this way you include all Merchants whether they have a transaction or not (which is what a LEFT JOIN does):
SELECT Merchant_Name, sum(transaction_amount) as `Transaction Amount`,
count(*) as `No. Of Transactions`
FROM Merchants
LEFT JOIN Transactions using (MERCHANT_NUMBER)
group by Merchant_Name
Of course you could instead change your query as written to a RIGHT JOIN (instead of a LEFT JOIN) but I find this just makes the code hard to read so I tend to avoid them.
Note your query as written does what you experienced; it shows all Transactions whether they have a merchant or not (FROM Transactions LEFT JOIN Merchants)
As I just noticed, your data is not normalized. You've got extra merchants in the transaction table that aren't accounted for in your merchants table. This will cause you lots of trouble.
You should find out what is doing this and prevent it, the best way is to remove the Merchant_Name column from your Transactions table altogether. If you have no control over the table designs, you should talk to someone who does :-P. Barring that, try:
SELECT Merchant_Name, sum(transaction_amount) as `Transaction Amount`,
count(*) as `No. Of Transactions`
FROM
(
SELECT Merchant_Name, Merchant_Number
FROM Merchants
UNION
SELECT Merchant_Name, Merchant_Number
FROM Transactions
) Real_Merchants
LEFT JOIN Transactions using (MERCHANT_NUMBER)
group by Merchant_Name

Answer 2

You almost had it, just swap Merchants and Transaction:
SELECT Merchant_Name, SUM(transaction_amount) AS 'Transaction Amount',
COUNT(*) AS 'No. Of Transactions'
FROM Merchants
LEFT JOIN Transactions using (MERCHANT_NUMBER)
GROUP BY Merchant_Name

Answer 3

When you want all the rows from a particular table in a LEFT JOIN you need to name that table first.
so FROM merchants LEFT JOIN transactions will give you what you want.

Answer 4

Change the order of your tables in the query - Merchants LEFT JOIN Transactions. Then, you will need to replace some NULL values with zeroes. Something like this:
SELECT Merchant_Name, ISNULL(SUM(transaction_amount), 0) AS 'Transaction Amount',
COUNT(*) AS 'No. Of Transactions'
FROM Merchants
LEFT JOIN Transactions using (MERCHANT_NUMBER)
GROUP BY Merchant_Name

Answer 5

You must normalize you database. Try this:
SELECT
MN.Merchant_Number,
MN.Merchant_Name,
ISNULL(SUM(T.Amount), 0) TransactionAmount,
COUNT(T.Merchant_Number) NoOfTransactions
FROM
(
SELECT Merchant_Number, Merchant_Name FROM Merchants
UNION
SELECT Merchant_Number, Merchant_Name FROM Transactions
) MN
LEFT JOIN Transactions T
ON MN.Merchant_Number = T.Merchant_Number
GROUP BY MN.Merchant_Number, MN.Merchant_Name

Answer 6

As DEFG does not appear in the left table ('Transactions') it will not appear in the result of the LEFT JOIN, which takes all the rows from the left table and finds them a match in the right. One option is to:
1. Switch between the tables and do Merchants LEFT JOIN Transactions.
2. Use COALESCE to handle the NULL values you're going to get for the right table (as you have no match for DEFG there), and replace it with 0.