您可以使用
CASE：
select concat(first_name, " ", last_name) as CustomerName, address, case when address2 is null then 'None Provided' else address2 end as address2, postal_pre from customerinner join address using (address_id);
甚至更好
COALESCE：
select concat(first_name, " ", last_name) as CustomerName, address, COALESCE(address2,'None Provided') as address2, postal_pre from customerinner join address using (address_id);
如果数据包含空字符串或null，那么我将考虑使用：
select concat(first_name, " ", last_name) as CustomerName, address, case when address2 is null or address2 = '' then 'None Provided' else address2 end as address2, postal_pre from customerinner join address using (address_id);
