Question

I’ve been having trouble finding a way to retrieve data – I need a way to traverse multiple foreign key relationships in a mysql query. The general table structure is as follows:

Subjects (table)
-id
-fname
-lname
-etc.

Events(table):
-id
-subject_id
-procedure_id
-date

Procedures(table):
-id
-description

These identifiers are used in other tables, such as:

Assessment1(table):
-id
-event_id
-data, etc.

Assessment2(table):
-id
-event_id
-data, etc.

The problem I’m having is in being able to pull all of the assessment data for subjects that have events for both Assessment 1 and Assessment 2. I’ve been able to successfully pull data for one assessment, or for two assessments that share the same event_id, in this example, Assessment1 and Assessment 1a. But I need a way to be able to link based on the subject_id, which is not in the assessment table as this goes through the Events table.

The query that I have used successfully (based off of event_id) is:

select 
    s.first_name, 
    s.last_name, 
    s.ssn, 
    e.subject_id, 
    n.*,
    l.Q23 
from 
    subjects s, 
    events e, 
    Assessment1 n, 
    Assessment1a l 
where 
    e.subject_id = s.id 
    and l.event_id = e.id 
    and l.Q23 = "1" 
    and n.event_id = e.id

How can I modify this to pull data for assessments with different event_ids but the same subject_ids?

Answer 1

How can I modify this to pull data for assessments with different event_ids but the same subject_ids?

I think this is what you need.

SELECT *
FROM subjects s
JOIN events e1 ON e1.subject_id = s.id
JOIN events e2 ON e2.subject_id = s.id and e1.id != e2.id
JOIN assessment1 a1 ON a1.event_id = e1.id
JOIN assessment2 a2 ON a2.event_id = e2.id