please show just those customers who's channel code was email
SELECT DISTINCT customer_name FROM customers AS t1 JOIN customer_contact_channels AS t2 ON t1.customer_id   =   t2.customer_id where channel_code  =  "Email"
customers whose channel code was email refers to filtering on channel_code = 'Email'; showing distinct names to avoid duplicates; involves joining customers table with customer_contact_channels table on customer_id.