Can you be a bit more specific in the description about what datatype the column "number" should be?

Perhaps this issue would benefit from being re-written with appropriate cases. It's a waste of time to work with inconsistent input, as the saying goes - "garbage in, garbage out." In reality, people work with actual data rather than just descriptions of it. When something goes wrong, it's necessary to provide the dataset that caused the issue. I find it amusing that many computer science challenges focus on "corner" cases. In my experience working on production problems, developers don't typically think about "corner" cases; they focus on addressing the specific problem at hand. While there may always be another "corner" case that arises, the sheer number of potential cases is unlimited, so the idea of covering them all is unrealistic.

Maybe the default test should include result validation? :-)

This problem contains the most retarded tests I have even seen. They should be re-written/fixed of this assignment should not be on this site.

Is something wrong with the simple test?

select * from visits;

id	entry_time	exit_time
1	2019-04-01 00:00:00 +0000	2019-04-03 00:00:00 +0000
2	2019-04-08 00:00:00 +0000	2019-04-09 00:00:00 +0000

The above obviously do not overlap, but the expected result is

when_happened	visits_count
2019-04-01 00:00:00 +0000	2

10.0.0.0 is not a valid address, it is a network address.

Good job! It seems like the only pure SQL solution that works.

Not working for the following simple test:

 id | user_id | name
----+---------+------
  1 |       1 | a
  2 |       2 | b
  3 |       1 | b
  4 |       2 | a

Not working for the following simple test:

 id | user_id | name
----+---------+------
  1 |       1 | a
  2 |       2 | b
  3 |       1 | b
  4 |       2 | a

This is not working for the following simple test:

 id | user_id | name
----+---------+------
  1 |       1 | a
  2 |       2 | b
  3 |       1 | b
  4 |       2 | a

Could the following test be added please?

 id | user_id | name
----+---------+------
  1 |       1 | a
  2 |       2 | b
  3 |       1 | b
  4 |       2 | a
(4 rows)

Expected result:

 user_id | name
---------+------
       1 | a
       2 | b

This query does not return correct answer for the following test:

 id | user_id | name
----+---------+------
  1 |       1 | a
  2 |       2 | b
  3 |       1 | b
  4 |       2 | a
(4 rows)

This query is not working. To prove it, just remove id = 4 from the test data:

 id | user_id | name
----+---------+------
  1 |       1 | a
  2 |       2 | b
  3 |       1 | b
  5 |       2 | a
  6 |       1 | a

The result will be:

 user_id | name
---------+------
       1 | a
       2 | a

There is a problem with the statement: "The user's first attempt to set their nickname is always successful". What if a user choses a nickname if it is already in use? Why the failed first attempt should not be recorded in the table. First attempt is still an attempt. If this taken into consideration some pure SQL solutions stop working.