Which bikes were live on December 8? - mysql

Question

I am new to SQL (MySql) and I got stuck with this problem with one of my assignments and couldn't get answer from anyone or searching online about the subject. The question goes like:
Which bikes were live (is_live = TRUE) on December 8? (Display only bike_id).
Note: The Table does not have a row entry for December 8, and that is as intended
Name Type Description
id int LOG ID(PK)
bike_id int id of the bike
is_live boolean flag to indicate whether bike is live (TRUE/FALSE)
updated_on Date Date on which status of bike was updated
BIKE_LIVE_LOG table:
id bike_id is_live updated_on
1 1 TRUE 2018-12-01
2 2 TRUE 2018-12-01
3 3 TRUE 2018-12-01
4 3 FALSE 2018-12-02
5 2 FALSE 2018-12-05
6 3 TRUE 2018-12-10
I couldn't move forward with the question as I am not even getting the approach for it with my current knowledge.
I have used this query to generate the the last_update_date grouped by each bike_id.
select bll.bike_id, max(bll.updated_on) as last_update_date
from bike_live_log as bll
where bll.updated_on <= '2018-12-08'
group by bll.bike_id;
The output will be 1.

Answer 1

I'll try to help you get to the last step. You're really, really close!
You were correct in going down the road of finding the most recent updated_on date for each bike_id. It doesn't matter how many times a bike has been turned on or off; you really only care about the most recent status prior to the date you're interested in.
With your current query, you already know when each bike_id was last updated prior to December 8th.
Next, you can use that information to find out what the is_live value was for each of those bike_id values as of that last_update_date.
You can do that by using your existing query as a sub-query, or a CTE if you prefer, and join back to your main table again. Your JOIN criteria will be bike_id to bike_id and updated_on to last_update_date. By joining on the dates, you'll only return a single record for each bike_id, and that record will be the one you're interested in.
After you have your JOIN put together, you'll just need to add a WHERE clause to limit your result set to the rows where is_live = 'TRUE', which will return just bike_id of 1.

Answer 2

Your requirement can be expressed more data-centrically as find bikes whose last known status on or before Dec 8 was live.
This is one way (IMHO the most readable way) to express that in SQL:
select bike_id
from bike_live_log bll
where updated_on = (
select max(updated_on)
from bike_live_log
where bike_id = bll.bike_id
and updated_on <= '2018-12-08'
)
and is_live
The (corelated) subquery finds the date of the last update on or before Dec 8 for the current row of the outer query. If there's no such row, null will be returned, which won't match any rows from the outer query so, only bikes that have data on or before Dec 8 will be returned.

Answer 3

tl;dr
Here is the MySQL code you need:
SELECT bike_id FROM (
SELECT * FROM (
SELECT * FROM bikes
WHERE updated_on < '20181209'
ORDER BY updated_on DESC, id DESC
) AS sub
GROUP BY bike_id
) AS sub2
WHERE is_live = true
Why does this work?
We need to break down the question a little bit. I like to start from a position of "how can I get the information I need in a format that makes sense to me (as a human)?".
So the first thing I did was to get a list of all bikes with updated_on dates before the 9th Dec (i.e. were updated on the 8th Dec or before). I also ordered this by updated_on field so I (as a human) could easily see the "latest" record which will tell me the most recent status of each bike on or before the 8th Dec:
SELECT * FROM bikes
WHERE updated_on < '20181209'
ORDER BY updated_on DESC, id DESC
From this I can see there are 5 records of bike status changes before the 9th Dec. I can easily see that for each bike there are multiple "update" records, but I can now start from the top of the list and each bike id I encounter is the status on the 8th Dec.
Additionally I included an order_by for the record id. This is necessary because there could be multiple updates per day. The date itself won't tell us which of those updates was the latest ON THE SAME DAY, so we use the ID to determine that. (assuming it's chronologically incremental).
Now we have a list of all statuses and bike ids in the database before the 9th Dec we need to limit this to only one record per bike. This is easy, we can wrap the original query with a new query with a Group By directive on the bike_id.
SELECT * FROM (
SELECT * FROM bikes
WHERE updated_on < '20181209'
ORDER BY updated_on DESC, id DESC
) AS sub
GROUP BY bike_id
Group By in MySQL selects the first record it comes across for each group. We have already ordered the list by date and id in the original query, so the first record for each bike_id will be the latest status for that bike as of the 8th Dec.
Now, all that's left for us to do is to select the bike_id and filter out non-live bikes by using WHERE is_live = true on this latest query. That's how we end up with the query at the start of this answer:
SELECT bike_id FROM (
SELECT * FROM (
SELECT * FROM bikes
WHERE updated_on < '20181209'
ORDER BY updated_on DESC, id DESC
) AS sub
GROUP BY bike_id
) AS sub2
WHERE is_live = true
I hope this helps.

Answer 4

SELECT
bike_id,
MAX(CASE WHEN last_live_date <= '2018-12-08' AND ( last_not_live_date < last_live_date OR last_not_live_date IS NULL) THEN 1 ELSE 0 END) AS final_status
FROM
(
SELECT
bike_id,
MAX(CASE WHEN is_live = TRUE THEN updated_on END) AS last_live_date,
MAX(CASE WHEN is_live = FALSE THEN updated_on END) AS last_not_live_date
FROM `BIKE_LIVE_LOG`
WHERE updated_on <= '2018-12-08'
GROUP BY bike_id
) AS a
GROUP BY bike_id
HAVING final_status = 1;

Answer 5

with A as (select * from bike_live_log
where updated_on <= '2018-12-08'),
B as (select bike_id,max(updated_on) as updated from A
group by bike_id)
select A.bike_id from A inner join B
on A.updated_on = B.updated
where is_live = True;