How to find the average of two columns but if one value is null, then only show me the other value?

I have a table where I want to find the average value of column1 and column2, but in some cases there may be null values in either columns. In that case, I just want to be shown the value that is NOT null.

Example:

So in this case I want to see:

I'm still a beginner so my knowledge is pretty limited, also I want to keep it as simple as possible. I'm using phpmyadmin if that makes any difference.

I've tried sum(table1+table2)/2 which results in null if one of the values is null. I've also tried

SELECT 
   CASE 
     WHEN table2 IS null 
       THEN SUM(table1) 
     ELSE SUM(table1+table2)/2 
   END

but it just returns the average of rows where there isn't a null value (I only get back 12, not the 10 as well (refer to the above table))

You are near with the case expression.

There is no need for sum and you have not considered null for column1 .

Fixing those, the query would be :

SELECT 
   CASE 
     WHEN Column1 IS null THEN Column2 
     WHEN Column2 IS null THEN Column1 
     ELSE (Column1+Column2) / 2 
   END as column3
FROM myTable

See example