How to relate a table more than one time in SQL? - mysql

Question

I want to relate two tables and a relation table. Tables are: Person with it's primary key named id_person, Activity with it's primary key named id_activity and a table that relates the previous two tables: Activity_Person that contain as primary and foreign keys id_activity and id_person.
To relate this tables using the old JOIN format this would work:
select * from activity, person, activity_person
where activity.id_activity = activity_person.id_activity and person.id_person = activity_person.id_person;
This would show the activities that each person has taken part on.
But now I'm learning about JOINs and I don't know what's the correct format to relate a table that appears twice (Activity_Person).
I have tried this:
select * from
person inner join activity_person on person.id_person = activity_person.id_person,
activity inner join activity_person on activity.id_activity = activity_person.id_activity;
But I get the following error:
Not unique table/alias: 'activity_person'
What's the correct format?

Answer 1

You don't need activity_person twice here. Just do
select *
from person
inner join activity_person on person.id_person = activity_person.id_person
inner join activity on activity.id_activity = activity_person.id_activity;

Answer 2

I think you just want two joins:
select *
from person p inner join
activity_person ap
on p.id_person = ap.id_person inner join
activity a
on a.id_activity = ap.id_activity;
I'm not sure why you are trying to repeat activity_person in your query.
Also note that table aliases make the query easier to write and to read.

Answer 3

Your syntax is incorrect.
select * from activity, person, activity_person
where activity.id_activity = activity_person.id_activity
and person.id_person = activity_person.id_person;
Is equivalent to :
select *
from person
inner join activity_person
on person.id_person = activity_person.id_person -- <- remove the comma there
inner join activity
on activity.id_activity = activity_person.id_activity;
Basically, the syntax is like this :
SELECT <the fields to select>
FROM <table name>
JOIN <table to join>
ON <joining condition>
-- if you want to add another table :
JOIN <new table to join>
ON <joining condition>