Which customers use email as the contact channel?
select * from customer_contact_channels where channel_code = 'Email'
What are the dates these customers became active?
select active_from_date from customer_contact_channels where channel_code = 'Email'
What are the name and active date of these customers?
select t1.customer_name, t2.active_from_date from customers as t1 join customer_contact_channels as t2 on t1.customer_id = t2.customer_id where t2.channel_code = 'Email'