SQL select last before by date and group by - mysql

Question

Using MySQL, I want to select all submissions (rows) which last submission is NULL and previous one is not, grouped by user_id.
If I consider the following example table, then the answer would be rows: 2, 5 and 7.
| ID | submission | date_submission | user_id |
|----|------------|-----------------|---------|
| 1 | lorem | 2019-01-01 | 1 |
| 2 | ipsum | 2019-01-02 | 1 |
| 3 | NULL | 2019-01-03 | 1 |
| 4 | amet | 2019-01-05 | 2 |
| 5 | NULL | 2019-01-06 | 2 |
| 6 | sit | 2019-01-04 | 1 |
| 7 | sed | 2019-01-08 | 3 |
| 8 | elit | 2019-01-07 | 4 |
| 9 | NULL | 2019-01-09 | 3 |
MRE:
CREATE TABLE submissions (
id int NOT NULL,
submission varchar(45) NULL,
date_submitted date NOT NULL,
user_id int DEFAULT NULL
);
insert into submissions (1, "lorem", 2019-01-01, 1);
insert into submissions (2, "ipsum", 2019-01-02, 1);
insert into submissions (3, NULL, 2019-01-03, 1);
insert into submissions (4, "amet", 2019-01-05, 2);
insert into submissions (5, NULL, 2019-01-06, 2);
insert into submissions (6, "sit", 2019-01-04, 1);
insert into submissions (7, "sed", 2019-01-08, 3);
insert into submissions (8, "elit", 2019-01-07, 4);
insert into submissions (9, NULL, 2019-01-09, 3);

Answer 1

First get the last date with null submission for each user and then join to the table to get the rows of the previous dates.
By using ROW_NUMBER() get the last of these previous dates if it is not null:
select t.id, t.submission, t.date_submitted, t.user_id
from (
select s.*,
row_number() over (partition by s.user_id order by s.date_submitted desc) rn
from submissions s inner join (
select user_id,
max(case when submission is null then date_submitted end) maxnulldate
from submissions
group by user_id
) g on g.user_id = s.user_id and g.maxnulldate > s.date_submitted
) t
where t.rn = 1 and t.date_submitted is not null
See the demo.
Results:
| id | submission | date_submitted | user_id |
| --- | ---------- | -------------- | ------- |
| 2 | ipsum | 2019-01-02 | 1 |
| 4 | amet | 2019-01-05 | 2 |
| 7 | sed | 2019-01-08 | 3 |
I guess you meant row number 4 and not 5 in your expected results, right?

Answer 2

You can use lag() for this:
select s.*
from (select s.*,
lag(submission) over (partition by user_id order by date_submitted) as prev_submission
from submissions s
) s
where prev_submission is not null and submission is null;
Here is a db<>fiddle.
EDIT:
It occurs to me that "last submission" really is the last submission for each user. In that case, the above can be tweaked:
select s.*
from (select s.*,
row_number() over (partition by user_id order by date_submitted desc) as seqnum,
lag(submission) over (partition by user_id order by date_submitted) as prev_submission
from submissions s
) s
where prev_submission is not null and submission is null and seqnum = 1;