Question

OK, so I have two tables I’m working with – project and service, simplified thus:

project
-------  
id PK  
name str

service  
------- 
project_id FK for project  
time_start int (timestamp)  
time_stop int (timestamp)  

One-to-Many relationship.

Now, I want to return (preferably with one query) a list of an arbitrary number of projects, sorted by the total amount of time spent at them, which is found by SUM(time_stop) - SUM(time_start) WHERE project_id = something.

So far, I have

SELECT project.name  
FROM service  
LEFT JOIN project ON project.id = service.project_id  
LIMIT 100

but I cannot figure out how what to ORDER BY.

Answer 1

You need a GROUP BY:

SELECT project.name
FROM service
LEFT JOIN project ON project.id = service.project_id
GROUP BY project_id
ORDER BY SUM(time_stop - time_start)
LIMIT 100