为默认调用的数据库字段分配默认值

I'm working on an attempted social networking site with a relative of mine as a side project. I'm writing most of it bymyself. I have made it so each user is stored into a table with their data (mysql) and i want to do a single compare statement so that if a field is blank to change the recieved variable to "No data".I'm working on an attempted social networking s