Two LEFT JOINs in SQL does not preserve data - mysql

Question

This query:
SELECT contacts.name, accounts.account
FROM contacts
LEFT JOIN deals
ON contacts.id = deals.contact_id
LEFT JOIN
accounts ON accounts.deal_id = deals.id;
returns:
+------+-------------------+
| name | account |
+------+-------------------+
| Bob | fun deal account |
| Bob | NULL |
| John | NULL |
+------+-------------------+
But I expected:
+------+-------------------+
| name | account |
+------+-------------------+
| Bob | fun deal account |
| Bob | fun deal account |
| John | NULL |
+------+-------------------+
The first LEFT JOIN behaves correctly. Since there are two deals for Bob, Bob correctly shows up twice in result set. But the second LEFT JOIN does not behave right, because the account should have been carried over twice for both Bob records, but instead there is a NULL for the second bob.
The schema:
CREATE TABLE contacts(
id int AUTO_INCREMENT,
name VARCHAR(50),
Primary Key(id)
)
INSERT INTO contacts VALUES('Bob');
INSERT INTO contacts(name) VALUES('John');
CREATE TABLE deals(
id int AUTO_INCREMENT,
name VARCHAR(20),
contact_id int,
FOREIGN KEY(contact_id) REFERENCES contacts(id),
Primary Key(id)
);
INSERT INTO deals(name, contact_id) VALUES('cool deal',1);
INSERT INTO deals(name, contact_id) VALUES('another cool deal',1);
CREATE TABLE accounts(
id int AUTO_INCREMENT,
account VARCHAR(50),
deal_id int,
FOREIGN KEY(deal_id) REFERENCES deals(id),
PRIMARY KEY (id)
)
INSERT INTO accounts(account, deal_id) VALUES('fun deal account', 1);
Why doesn't the second LEFT JOIN give desired behavior and how can I get the 'fun deal account' account to show up for both Bobs?

Answer 1

Bob have two deals but deals.id is auto_increment so fun deal account only match the first row in deals table, the cool deal.
You need to add INSERT INTO accounts(account, deal_id) VALUES('fun deal account', 2); too

Answer 2

In case of doubts, decompose your query.
The first LEFT JOIN could be this:
SELECT contacts.id as contact_id, contacts.name, deals.id as deals_id, deals.name
FROM contacts
LEFT JOIN deals ON contacts.id = deals.contact_id
Which results in :
contact_id name deals_id name
1 Bob 1 cool deal
1 Bob 2 another cool deal
2 John NULL NULL
The second LEFT JOIN is:
LEFT JOIN accounts ON accounts.deal_id = deals.id
So the result given is logical given your data, you have only one account with deal_id=1 so it matches the first row where deals.id=1 : "cool deal" .
I think your mistake is on the last part of your query, the query you wanted is :
SELECT contacts.name, accounts.account FROM contacts LEFT JOIN deals ON contacts.id = deals.contact_id LEFT JOIN accounts ON accounts.deal_id = deals.contact_id
"accounts.deal_id = deals.contact_id" instead of "accounts.deal_id = deals.id" is the deal (pun intended) to have your expected result.