How to find NOT NULL columns by union? - mysql

Question

I have three table as:
1.table person
id, others
1, x
2, y
3, z
4, w
2.table followup, FOREIGN KEY (person_id) REFERENCES person (id)
id, person_id, ftime, details
1, 1, '2018-01-12', 'json_1'
2, 1, '2018-04-21', 'json_2'
3, 2, '2017-12-16', 'json_3'
4, 2, '2018-01-17', NULL
5, 3, '2018-06-02', 'json_5'
6, 4, '2018-01-19', NULL
3.table followup_track, FOREIGN KEY (fid) REFERENCES followup (id)
id, fid, ftime, details
1, 1, '2018-01-27', 't_json_1'
2, 2, '2018-05-07', 't_json_2'
3, 5, '2018-06-17', 't_json_3'
Now I want find all the last ftime of every person, and the details IS NOT NULL in followup and/or followup_track.
the result what I want to get is (here the pid is person's id):
pid, ftime, details
1, '2018-05-07', 't_json_2'
2, '2017-12-16', 'json_3'
3, '2018-06-17', 't_json_3'
Because there is no detalis != NULL for person.id = 4, so the result no need for pid=4.
Because the last time of person.id = 1 is '2018-05-07', so need that column.
I create a view like:
CREATE VIEW view_full_flup AS
SELECT
p.id AS pid, fp.ftime, fp.details
FROM
((followup_track fp
LEFT JOIN followup ON (fp.fid = followup.id))
LEFT JOIN person p ON (followup.person_id = p.id))
WHERE
fp.details IS NOT NULL
UNION
SELECT
f.person_id AS pid, f.ftime, f.details
FROM
followup f
WHERE
f.details IS NOT NULL
Then, I use sql:
SELECT *, MAX(`ftime`) FROM view_full_flup GROUP BY pid;
Is my solution right please? The details can not make index, and it is slow. How to do this right please?

Answer 1

You can’t select star group by(well, you might be able to in MySQL depending on how it’s set up but it’s not a great habit to get into for portability of sql skills), you have to specify a list of columns in your select and either group them (put the column name in the group by) or aggregate them (pass the column name into an aggregate function)
It feels like you’ve got your left join logic backwards- person is the only table you know you have records in, so it should be on the left side of the join.. the other tables are potentially recordless
We’re this my query I’d write it something more like:
select
Pid,
Max(case when ft.ftime > f.ftime then ft.ftime else f.ftime end) maxft
From
Person p
Left join (select * from followup where details is not null) f on f.person_id = p.id
Left join (select * from follup_track where details is not null) ft on ft.fid = f.id
Group by pid
We just join the set of tables once, having already filtered for records where either table’s details are present and then get the max date from either table
Note that this query can return null dates, if you have records where the date column is null even though details is filled in. If those are undesirable, filter hem with a HAVING or wrap the whole thing in another select and filter using a WHERE
Ps; as it stands, this query doesn’t seem particularly useful because though you know the most recent date you don’t have any other data. If it’s all you wanted to know (I wasn’t able to tell from your original post because the description and the query disagreed in what they said they wanted) then great, but if you wanted the other info, maybe using mysql 8’s new analytic queries would better suit:
Select * from
(
select
Pid,
Row_number over(partition by pid order by (case when ft.ftime > f.ftime then ft.ftime else f.ftime end) desc) rown
From
Person p
Left join (select * from followup where details is not null) f on f.person_id = p.id
Left join (select * from follup_track where details is not null) ft on ft.fid = f.id
) a where rown = 1