COALESCE

In most of DB engine you can find COALESCE function. It is helpful where you need to work with NULL values.

source table

mysql> select * from INFO;
+----+-------------+--------------+
| ID | GALLERY     | PHOTOGRAPHER |
+----+-------------+--------------+
|  1 | Cars        | Tom          | 
|  2 | Countryside | Petr         | 
|  3 | City        | Jan          | 
|  4 | Sport       | NULL         | 
+----+-------------+--------------+

When you need to return not NULL report, you can use COALESCE

COALESCE function

mysql> select GALLERY, coalesce(PHOTOGRAPHER,"unknown") as PHOTOGRAPHER from coalesce;
+-------------+--------------+
| GALLERY     | PHOTOGRAPHER |
+-------------+--------------+
| Cars        | Tom          | 
| Countryside | Petr         | 
| City        | Jan          | 
| Sport       | unknown      | 
+-------------+--------------+