Query returning too many results - sql

Question

SQL query that returns expected 29 results for a.id = 366
select a.name, c.name, MAX(B.date), MAX(b.renew_date) as MAXDATE
from boson_course c
inner join boson_coursedetail b on (c.id = b.course_id)
inner join boson_coursedetail_attendance d on (d.coursedetail_id = b.id)
inner join boson_employee a on (a.id = d.employee_id)
where a.id = 366
GROUP BY a.name, c.name
order by MAX(b.renew_date), MAX(b.date) desc;
SQL code below that returns 34 results, multiple results where two different Provides supplied the same course. I know these extra results are because I added e.name to the list to be returned. But all that is needed is the 29 entries with the latest date and Providers names.
select a.name, c.name, e.name, MAX(B.date), MAX(b.renew_date) as MAXDATE
from boson_course c
inner join boson_coursedetail b on (c.id = b.course_id)
inner join boson_coursedetail_attendance d on (d.coursedetail_id = b.id)
inner join boson_employee a on (a.id = d.employee_id)
inner join boson_provider e on b.provider_id = e.id
where a.id = 366
GROUP BY a.name, c.name, e.name
order by MAX(b.renew_date), MAX(b.date) desc;
Can anyone rework this code to return a single DISTINCT Provider name with the MAX(renew_date) for each course.

Answer 1

This returns exactly one row per distinct combination of (a.name, c.name):
The one with the latest renew_date.
Among these, the one with the latest date (may differ from global max(date)!).
Among these, the one with the alphabetically first e.name:
SELECT DISTINCT ON (a.name, c.name)
a.name AS a_name, c.name AS c_name, e.name AS e_name
, b.renew_date, b.date
FROM boson_course c
JOIN boson_coursedetail b on c.id = b.course_id
JOIN boson_coursedetail_attendance d on d.coursedetail_id = b.id
JOIN boson_employee a on a.id = d.employee_id
JOIN boson_provider e on b.provider_id = e.id
WHERE a.id = 366
ORDER BY a.name, c.name
, b.renew_date DESC NULLS LAST
, b.date DESC NULLS LAST
, e.name;
The result is sorted by a_name, c_name first. If you need your original sort order, wrap this in a subquery:
SELECT *
FROM (<query from above>) sub
ORDER BY renew_date DESC NULLS LAST
, date DESC NULLS LAST
, a_name, c_name, e_name;
Explanation for DISTINCT ON:
Select first row in each GROUP BY group?
Why DESC NULL LAST?
PostgreSQL sort by datetime asc, null first?
Aside: Don't use basic type names like date ad column names. Also, name is hardly ever a good name. As you can see, we have to use aliases to make this query useful. Some general advice on naming conventions:
How to implement a many-to-many relationship in PostgreSQL?

Answer 2

Try using distinct on:
select distinct on (a.name, c.name, e.name), a.name, c.name, e.name,
B.date, b.renew_date as MAXDATE
from boson_course c
inner join boson_coursedetail b on (c.id = b.course_id)
inner join boson_coursedetail_attendance d on (d.coursedetail_id = b.id)
inner join boson_employee a on (a.id = d.employee_id)
inner join boson_provider e on b.provider_id = e.id
where a.id = 366
ORDER BY a.name, c.name, e.name, B.date desc
order by MAX(b.renew_date), MAX(b.date) desc;