table creation logic - mysql

Question

I am looking to take two tables I have a perform a data transformation to create a single table. I have an events table and user table:
Events: {id, user_id, start_date, end_date, cost...}
Users: {id, name, ...}
I am trying to create a table at that shows user spend at a daily level, assuming the user start with a starting cost of zero and it goes up after every event.
The intended output would be:
{date, userid, beginning_balance, sum(cost), num_of_events}
i need some direction on how to tackle this one as I am not very familiar with data transformation within SQL

Answer 1

Your requirement is a bit unclear but you may be after something like this
drop table if exists event;
create table event(id int auto_increment primary key, user_id int,start_date date, end_date date, cost int);
insert into event (user_id,start_date , end_date, cost) values
(1,'2017-01-01','2017-01-01',10),(1,'2017-01-01','2017-01-01',10),
(1,'2017-02-01','2017-01-01',10),
(2,'2017-01-01','2017-01-01',10);
select e.user_id,start_date,
ifnull(
(select sum(cost)
from event e1
where e1.user_id = e.user_id and e1.start_date <e.start_date
), 0 )beginning_balance,
sum(cost),count(*)
as num_of_events
from users u
join event e on e.user_id = u.userid
group by e.user_id,start_date
+---------+------------+-------------------+-----------+---------------+
| user_id | start_date | beginning_balance | sum(cost) | num_of_events |
+---------+------------+-------------------+-----------+---------------+
| 1 | 2017-01-01 | 0 | 20 | 2 |
| 1 | 2017-02-01 | 20 | 10 | 1 |
| 2 | 2017-01-01 | 0 | 10 | 1 |
+---------+------------+-------------------+-----------+---------------+
3 rows in set (0.03 sec)

Answer 2

Can you try this query
SELECT
data,
User.id AS userid,
0 AS beginning_balance,
SUM(cost) AS cost,
COUNT(0) AS num_of_events
FROM
Users
LEFT JOIN Events ON (user_id = Users.id)
GROUP BY
Users.id