MYSQL HELP - Order of Operations and Sub Query - mysql

Question

There are two tables I am working with (ft_form_1 & ft_field_options).
ft_form_1
submission_id facility_id admits reporting_date timestamp
1 111A 1 2017-03-31 00:00 2017-03-31 17:53:17
2 222B 3 2017-03-31 00:00 2017-03-31 18:42:20
3 333C 6 2017-03-31 00:00 2017-03-31 19:27:47
4 222B 0 2017-04-01 00:00 2017-04-01 18:12:12
5 333C 4 2017-03-31 00:00 2017-04-01 19:38:25
6 333C 5 2017-04-01 00:00 2017-04-01 20:31:16
ft_field_options
list_id option_order option_value option_name
1 4 111A New York
1 2 222B Chicago
1 1 333C Boston
1 3 444D Miami
What I am currently getting:
facility_id option_name option_order admits reporting_date timestamp
111A New York 3 1 2017-03-31 00:00 2017-03-31 17:53:17
What I want to get:
facility_id option_name option_order admits reporting_date timestamp
111A New York 3 1 2017-03-31 00:00 2017-03-31 17:53:17
222B Chicago 2 3 2017-03-31 00:00 2017-03-31 18:42:20
333C Boston 1 4 2017-03-31 00:00 2017-04-01 19:38:25
With the below query, I am trying to get a list of all submissions from a specified 'reporting_date' for each 'facility_id' with a 'list_id' equal to 1. If multiple submissions are sent on the same 'reporting-date', only the submission with the most recent 'timestamp' will be shown.
Problem: I believe this query is not running in the order I'd like it to. It seems the query is finding the max 'timestamp' for each 'facility_id' in the table and then filtering to only show submissions with a specified 'reporting_date'. I'd like that to occur in reverse order - where the query filters to only show submissions with a specified 'reporting_date' and then narrows the list down to only show the max 'timestamp' for each 'facility_id. As you can see from my above example, Chicago and Boston are being left off the list because they have a more recent 'timestamp' on the following day.
I am a MYSQL newbie, so any help is appreciated! I originally got the idea of the MAX subquery from this link: http://www.w3resource.com/sql/aggregate-functions/max-date.php
SELECT t1.facility_id, t1.admits, t1.reporting_date, t1.timestamp,
t2.option_name, t2.option_order
FROM ft_form_1 t1
LEFT JOIN ft_field_options t2
ON (t1.facility_id = t2.option_value AND t2.list_id = 1)
WHERE (DATE_FORMAT(t1.reporting_date,'%m/%d/%Y') =
DATE_FORMAT(NOW() - INTERVAL 1 DAY,'%m/%d/%Y')) AND (timestamp=(
SELECT MAX(timestamp)
FROM ft_form_1
WHERE facility_id = t1.facility_id))
ORDER BY option_order ASC
*Imagine today is April 1st 2017 for the above query to work.

Answer 1

The answer here is to use the structured part of structured query language. SQL is a declarative, not procedural, language, so it can be confusing to think about the order in which it does things.
First, you are looking for rows with a list_id of 1. Let's do a query to get that. Notice the use of JOIN rather than LEFT JOIN: we don't want the unmatched elements. (http://sqlfiddle.com/#!9/8bdc3c/1/0)
SELECT t1.*
FROM ft_form_1 t1
JOIN ft_field_options t2
ON t1.facility_id = t2.option_value
AND t2.list_id = 1
Next we need to find the timestamp of the last submission on each reporting date for each facility. (http://sqlfiddle.com/#!9/8bdc3c/3/0)
SELECT MAX(timestamp) timestamp,
reporting_date reporting_date,
facility_id
FROM ft_form_1
GROUP BY reporting_date, facility_id
These kinds of subqueries are a good way to go, because they're pretty easy to test.
Finally, we join these two subqueries together to get what we want. (http://sqlfiddle.com/#!9/8bdc3c/6/0)
SELECT a.*
FROM (
SELECT t1.*
FROM ft_form_1 t1
JOIN ft_field_options t2
ON t1.facility_id = t2.option_value
AND t2.list_id = 1
) a
JOIN (
SELECT MAX(timestamp) timestamp,
reporting_date reporting_date,
facility_id
FROM ft_form_1
GROUP BY reporting_date, facility_id
) b ON a.facility_id = b.facility_id
AND a.timestamp = b.timestamp
AND a.reporting_date = b.reporting_date
The trick is in the GROUP BY subquery, that finds the last timestamp for each reporting date.
Now, you have a submission_id in your table. If that submission_id is autoincrementing, then it's likely that the largest submission_id value for reporting data in each facility is the one you want. If that's true you can simplify the query a whole lot.
This gets you the submission_id valued that correspond to the submissions you want
SELECT MAX(submission_id)
FROM ft_form_1
GROUP BY reporting_date, facility_id
You can join that to your main tables like this, to get the result you want. (http://sqlfiddle.com/#!9/8bdc3c/10/0)
SELECT t1.*
FROM ft_form_1 t1
JOIN ft_field_options t2 ON t1.facility_id = t2.option_value
AND t2.list_id = 1
JOIN (
SELECT MAX(submission_id) submission_id
FROM ft_form_1
GROUP BY reporting_date, facility_id
) q ON t1.submission_id = q.submission_id