MySQL - Join and conditional select columns - mysql

Question

Don't know how to excatly name the problem. Therefore my example tables:
People:
id_people | peopleName |
------------------------
1 | John |
2 | Jane |
3 | Frank |
4 | Emma |
Courses:
id | id_course | id_people | date |
-----------------------------------------
1 | 1 | 1 | 2011-02-14 |
2 | 2 | 1 | 2013-05-01 |
3 | 3 | 1 | 2015-08-25 |
4 | 1 | 2 | 2012-01-05 |
5 | 1 | 4 | 2017-10-10 |
Now I want to generate a table where I have one row per person and for every course a column with the date like:
id_people | peopleName | Course1 | Course2 | Course3 |
---------------------------------------------------------------
1 | John | 2011-02-14 | 2013-05-01 | 2015-08-25 |
2 | Jane | 2012-01-05 | | |
3 | Frank | | | |
4 | Emma | 2017-10-10 | | |
I tried following query:
SELECT *,
max(case when courses.id_course = 1 then cource.date end) as Course1,
max(case when courses.id_course = 2 then cource.date end) as Course2,
max(case when courses.id_course = 3 then cource.date end) as Course3
FROM people
JOIN courses ON people.id_people = courses.id_people
GROUP BY people.id_people
But I got the error #1055 ([..] this is incompatible with sql_mode=only_full_group_by).
So, it forces me to add the column course.id, course.id_course and course.date to the group by statemant. This results in a row per per people and course.
How can I get my desired resulting table with a sql statment?
I do not have the privilege to change the sql_mode variable.

Answer 1

So it forces me to add the column course.id, course.id_course and
course.date to the group by statemant...
No it does not.
You only need to change the join to a LEFT JOIN and group by people.id_people, people.peopleName and you will get the results that you want:
SELECT people.id_people, people.peopleName,
max(case when courses.id_course = 1 then courses.date end) as Course1,
max(case when courses.id_course = 2 then courses.date end) as Course2,
max(case when courses.id_course = 3 then courses.date end) as Course3
FROM people
LEFT JOIN courses ON people.id_people = courses.id_people
GROUP BY people.id_people, people.peopleName
See the demo.
Results:
| id_people | peopleName | Course1 | Course2 | Course3 |
| --------- | ---------- | ----------- | ----------- | ----------- |
| 1 | John | 2011-02-14 | 2013-05-01 | 2015-08-25 |
| 2 | Jane | 2012-01-05 | | |
| 3 | Frank | | | |
| 4 | Emma | 2017-10-10 | | |

Answer 2

You haven't define id_people to be a primary key or at least unique. You should probably do that.
In the meantime, you can aggregate first and then join:
SELECT *
FROM people p JOIN
(SELECT c.id_people
max(case when c.id_course = 1 then c.date end) as Course1,
max(case when c.id_course = 2 then c.date end) as Course2,
max(case when c.id_course = 3 then c.date end) as Course3
FROM courses c
GROUP BY c.id_people
) c
USING (id_people);