How to get most recent balance from many users balances? - mysql

Question

I have two table users and transactions. transactions table has relation with users two table format like below
users
id name email created
1 a a#mail.com 12-03-01
2 b b#mail.com 11-03-01
Transactions
id user_id balance
1 1 250
2 1 550
3 2 50
4 2 1000
I need last inserted users balance from transactions table with all users information. I am new in sql.
So I have tried below code
select * from transactions
where id in (select max(id) from transactions group by user_id)
INNER JOIN users on transactions.user_id=users.id
It's giving me syntax error near inner join.Have I made any mistake in inner join ? or I am in wrong direction ?

Answer 1

If you only want the balance, then a correlated subquery might be faster:
select u.*,
(select t.balance
from transactions t
where t.user_id = u.id
order by t.id desc
limit 1
) as MostRecentBalance
from users u;
For maximum performance, you want an index on transactions(user_id, id desc, balance).
The reason this is faster is because it avoids the aggregation on the entire transactions table. This is even more important if you are only selecting a subset of users.
EDIT:
I originally read this question as one row per user. However, if you only want one row returned -- for the last insert into transactions -- then a simpler method is:
select u.*, t.balance
from users u join
transactions t
on u.id = t.user_id
order by t.id desc
limit 1;

Answer 2

The JOIN should be part of the FROM statement so it should look more like the code below.
select *
from transactions ts
INNER JOIN users
ON (transactions.user_id=users.id)
where ts.id in
(
select max(transactions.id)
from transactions
group by user_id
);
edited to clarify which id is in use as per Gordons suggestion

Answer 3

2 simple methods.
A sub query to get the lastest transaction, and from that all the transaction details and then the user
SELECT users.*
FROM users
INNER JOIN transactions
ON users.id = transactions.user_id
INNER JOIN
(
SELECT MAX(id) AS max_id
FROM transactions
) sub0
ON transactions.id = sub0.max_id
Or you could try ordering by the id descending with a limit of 1:-
SELECT users.*
FROM users
INNER JOIN transactions
ON users.id = transactions.user_id
ORDER BY transactions.id DESC
LIMIT 1
EDIT
To get the last transaction for all users then you could use the following:-
SELECT *
FROM users
INNER JOIN transactions
ON users.id = transactions.user_id
INNER JOIN
(
SELECT user_id, MAX(id) AS max_id
FROM transactions
GROUP BY user_id
) sub0
ON transactions.id = sub0.max_id
ON transactions.user_id = sub0.user_id