include only the first and last groups in query results - mysql

Question

Given the schema
The following query
SELECT a.user_id,
a.id,
a.date_created,
avg(ai.level) level
FROM assessment a
JOIN assessment_item ai ON a.id = ai.assessment_id
GROUP BY a.user_id, a.id;
Returns these results
user_id, a.id, a.date_created, level
1, 99, "2015-07-13 18:26:00", 4.0000
1, 98, "2015-07-13 19:04:58", 6.0000
13, 9, "2015-07-13 18:26:00", 2.0000
13, 11, "2015-07-13 19:04:58", 3.0000
I would like to change the query such that only the earliest results is returned for each user. In other words, the following should be returned instead
user_id, a.id, a.date_created, level
1, 99, "2015-07-13 18:26:00", 4.0000
13, 9, "2015-07-13 18:26:00", 2.0000
I think I need to add a HAVING clause, but I'm struggling to figure out the exact syntax.

Answer 1

I have done something like this, except for a small difference I wanted first 5 per group. The usage case was for reporting - means time for running query / creation of temp table was not a constraint.
The solution I had:
Create a new table with columns as id( a reference to original table) and id can be unique/primary
INSERT IGNORE INTO tbl1 (id) select min(id) from original_tbl where id not in (select id from tbl1) group by user_id
Repeat step 2 as many times you required( in my case it was 5 times). the new table table will have only the ids you want to show
Now run a join on tbl1 and original table will give you the required result
Note: This might not be the best solution, but this worked for me when I had to share the report in 2-3hours in a weekend. And the data size I had was around 1M records

Answer 2

Disclaimer: I am in a bit of a hurry, and have not tested this fully
-- Create a CTE that holds the first and last date for each user_id.
with first_and_last as (
-- Get the first date (min) for each user_id
select a.[user_id], min(a.date_created) as date_created
from assessment as a
group by a.[user_id]
-- Combine the first and last, so each user_id should have two entries, even if they are the same one.
union all
-- Get the last date (max) for each user_id
select a.[user_id], max(a.date_created)
from assessment as a
group by a.[user_id]
)
select a.[user_id],
a.id,
a.date_created,
avg(ai.[level]) as [level]
from assessment as a
inner join assessment_item as ai on a.id = ai.assessment_id
-- Join with the CTE to only keep records that have either the min or max date_created for each user_id.
inner join first_and_last as fnl on a.[user_id] = fnl.[user_id] and a.date_created = fnl.date_created
group by a.[user_id], a.id, a.date_created;