MYSQL Left Join on multiple table but select only most recent rows - mysql

Question

I have three tables (form_completions, customer_sessions and conversion_sessions) which are design like the below, these are heavily striped down for the purpose of this example:
Form_completions:
id | name | email
------------------------
101 | Tom | tom#website.com
102 | Ben | ben#website.com
Customer_sessions:
id | customer_id | session_id | session_source
---------------------------------
1 | 900 | 9kc73bsf | twitter
2 | 901 | 15jvuw83 | google
3 | 901 | 45h73bgf | twitter
Conversion_sessions:
id | customer_id | session_id | form_completion_id
------------------------------------
1 | 900 | 9kc73bsf | 101
2 | 901 | 45h73bgf | 102
The query that i currently have is:
SELECT custsess.session_source,
c.id as form_conversion_id,
c.name,
FROM conversion_sessions convsess
LEFT JOIN form_completions fc
ON convsess.`form_completion_id` = fc.`id`
LEFT JOIN customer_sessions custsess
ON custsess.`customer_id` = convsess.`customer_id`
This will give me the following:
session_source | form_conversion_id | name
------------------------------------------
twitter | 101 | Tom
google | 102 | Ben
twitter | 102 | Ben
But what i need is to avoid the duplication of form_conversion_id and only include the most recent so it would be...
session_source | form_conversion_id | name
------------------------------------------
twitter | 101 | Tom
twitter | 102 | Ben
Hopefully this makes sense.

Answer 1

Solution to your problem:
SELECT custsess.session_source,
fc.id as form_conversion_id,
fc.name
FROM conversion_sessions AS convsess
INNER JOIN form_completions AS fc
ON convsess.form_completion_id = fc.id
INNER JOIN customer_sessions AS custsess
ON custsess.customer_id = convsess.customer_id
AND custsess.session_id = convsess.session_id
While joining you forgot to join customer_sessions and coversion_sessions ON session_id.
OUTPUT:
session_source form_conversion_id name
twitter 01 Tom
twitter 102 Ben
For demo follow the below link:
https://www.db-fiddle.com/f/pQG4VCWAnoGtRJa6MkvST6/0

Answer 2

You need a Subquery to get the most recent, change your JOIN to something like this
LEFT JOIN customer_sessions custsess
ON custsess.id =
(
SELECT MAX(id)
FROM Customer_sessions
WHERE c.id = custsess.customer_id
)

Answer 3

Looking to your data You should just use inner join
You should use inner join on a subquery for max session_id
SELECT custsess.session_source,
c.id as form_conversion_id,
c.name,
FROM conversion_sessions convsess
customer_sessions custsess ON custsess.`customer_id` = convsess.`customer_id`
INNER JOIN (
select customer_id, max(session_id) as max_sess
from Customer_sessions
group by customer_id
) t on t.customer_id = custsess.customer_id and t.max_sess = custsess.session_id
INNER JOIN form_completions fc ON convsess.`form_completion_id` = fc.`id`