MySQL联接查询与IF条件

Question

I have 2 tables Customer & Company

COMPANY

COMP_ID | COMP_NAME  
1       | Google      
2       | Facebook    

CUSTOMER

CUST_ID | COMP_ID | CUST_NAME    
1       | 1       | John         
2       | 2       | NULL          
3       | 2       | Rob           

I want to write a query to display CUST_NAME as CONTACT but if CUST_NAME is NULL , then display COMP_NAME as CONTACT

Answer 1

Use COALESCE :

select cs.cust_id, coalesce(cs.cust_name, co.comp_name) contact
from customer cs
inner join company co
on cs.comp_id = co.comp_id;

If you have got empty string in cust_name do this:

select cs.cust_id, coalesce(nullif(cs.cust_name,''), co.comp_name) contact
from t_customer cs
inner join t_company co
on cs.comp_id = co.comp_id;

Live Demo

Answer 2

You can also achieve same by if or case

 select IF(cs.CUST_NAME IS NULL or cs.CUST_NAME = '', co.comp_name,cs.cust_name) as contact,
case when cs.CUST_NAME IS NULL or cs.CUST_NAME = '' then co.comp_name else cs.cust_name end as usingcasecontact
from customer cs
inner join  company co
on cs.comp_id = co.comp_id;

check its fiddle