SQL select command SUM across 3 related tables - sql

Question

I've changed my DB structure to make it more future proof. Now I'm having trouble with the new select query.
I have table called activities that has a list of activities and how many steps per minute that activity was worth. The table was structred like this:
Activities
id act_name act_steps
12 Boxing 250
14 Karate 300
17 Yoga 89
I have another table called distance that is structed like this:
Distance
id dist_activity_id dist_activity_duration member_id
1 12 60 12
2 14 90 12
3 17 30 12
I have the query that would SUM and produce a total for all activities in the distance table
SELECT ROUND(SUM(act_steps * dist_activity_duration / 2000),2) AS total_miles
FROM distance,
activities
WHERE activities.id = distance.dist_activity_id
This worked fine.
To future proof it incase the number of steps for an activity changes I've setup a table called steps that is structured like this:
Steps
id activity_steps
1 6
2 250
3 300
4 89
I then updated the activities table, removing the act_steps column and replacing it with steps_id so it now looks like this:
Updated activities
id act_name steps_id
12 Boxing 2
14 Karate 3
17 Yoga 4
I'm not sure how to create the select command to get the SUM using the new structure.
Could someone please help me with this?
Thanks
Wayne

Answer 1

Learn to use proper JOIN syntax! Your query should look like:
SELECT ROUND(SUM(a.act_steps * d.dist_activity_duration / 2000), 2) AS total_miles
FROM distance d JOIN
activities a
ON a.id = d.dist_activity_id;
If you need to lookup the steps, then add another JOIN:
SELECT ROUND(SUM(s.activity_steps * d.dist_activity_duration / 2000), 2) AS total_miles
FROM distance d JOIN
activities a
ON a.id = d.dist_activity_id JOIN
steps s
ON s.id = a.steps_id;