मैं sql क्वेरी में खाली फ़ील्ड में टेक्स्ट कैसे प्रदर्शित करूं?

आप CASE . का उपयोग कर सकते हैं :

select concat(first_name, " ", last_name) as CustomerName, 
   address, 
   case when address2 is null then 'None Provided' else address2 end as address2, 
   postal_code 
from customer
inner join address 
using (address_id);

या और भी बेहतर COALESCE :

select concat(first_name, " ", last_name) as CustomerName, 
   address, 
   COALESCE(address2,'None Provided') as address2, 
   postal_code 
from customer
inner join address 
using (address_id);

यदि डेटा में एक खाली स्ट्रिंग या नल है, तो मैं इसका उपयोग करने पर विचार करूंगा:

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_code 
from customer
inner join address 
using (address_id);