CREATE TABLE customer_contact_channels (customer_id VARCHAR2)
CREATE TABLE customers (customer_id VARCHAR2, customer_name VARCHAR2)

Which contact channel has been used by the customer with name 'Tillman Ernser'?
SELECT DISTINCT channel_code FROM customers AS t1 JOIN customer_contact_channels AS t2 ON t1.customer_id = t2.customer_id WHERE t1.customer_name = "Tillman Ernser"
SELECT DISTINCT "channel_code" FROM "customers" "t1" JOIN "customer_contact_channels" "t2" ON "t1"."customer_id" = "t2"."customer_id" WHERE "Tillman Ernser" = "t1"."customer_name"