Cách tạo nhiều hàng từ một hàng ban đầu

Sử dụng cái này cho một ID cụ thể

select id, city_name from(
    select id, city_1 as city_name from yourTable    
    union all
    select id, city_2 from yourTable    
    union all
    select id, city_3 from yourTable    
    union all
    select id, city_4 from yourTable
) as t where id= yourID

http://sqlfiddle.com/#!9/7ee1f/1

Sử dụng cái này cho toàn bộ bảng

 select id, city_name from(
    select id, city_1 as city_name from yourTable    
    union all
    select id, city_2 from yourTable    
    union all
    select id, city_3 from yourTable    
    union all
    select id, city_4 from yourTable
) as t
order by id