跨3个相关表的SQL选择命令SUM

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;