customer_id,email_address,first_name,last_name,birth_date,membership_level,country_id,created_at
1,john.doe@example.com,John,Doe,1985-05-15,GOLD,1,2023-01-05
2,jane.smith@gmail.com,Jane,Smith,1990-10-20,SILVER,2,2023-02-10
3,kid@outlook.com,Billy,Kid,2015-01-01,BRONZE,1,2023-03-01
4,,NoEmail,User,1970-01-01,PLATINUM,3,2022-12-15
5,duplicate@example.com,First,Dup,1980-01-01,GOLD,1,2023-01-01
6,duplicate@example.com,Second,Dup,1980-01-01,GOLD,1,2023-01-10
7,invalid-email,Bad,Email,1990-01-01,BRONZE,2,2023-04-10
8,old@timer.com,Old,Timer,1940-01-01,INVALID_TIER,1,2021-05-05
9,opted@out.com,Opted,Out,1988-08-08,GOLD,1,2023-06-06
