sql.md

SQL

Schema

clients
 - id
 - email

client_plans
 - id
 - client_id
 - plan_id
 - date_started
 - date_ended

plans
 - id
 - name
 - price

Sample data

clients:
 1,"[email protected]"
 
client_plans:
 1,1,7,2013-01-01,2013-01-03
 2,1,8,2013-01-03,2013-01-06
 3,1,7,2013-01-07,null

plans:
 7,"small",$10
 8,"large",$50

clients with a client_plans record with a null date_ended are still on that plan

1. Give me all the ids of clients who at some point (now or in the past) were on plan_id 7
2. Give me the email addresses of clients currently on plan_id 7