# 返回或提取字符串前n个字符 left(str,n)
# 字符串拼接 concat(str1,str2,...)
# 字符串字母全部大写化 upper(str)
select
    cust_id,
    cust_name,
    upper(concat(left(cust_contact,2),left(cust_city,3))) as user_login
from Customers
;