combining multiple columns

When you have something like this:

column1 column2 column3
null null hey
null hey null

anyway to make it just return one column by merging these 3 together?

Your main problem here is the NULL values; concatenating them will return NULL regardless of the other column contents.

Now the simplest solution would be to change the columns so that they store an empty string instead of NULL: you would just need to do this then

$sql = "SELECT CONCAT(col1, col2, col3) AS col";

If the NULL values are important elsewhere in your app then you will need to use IF to make this work

$sql = "SELECT CONCAT(IF(col1, col1, ''), IF(col2, col2, ''), IF(col3, col3, '')) AS col";

That works because NULL is false so it will be replaced by the empty string ''.

The columns come from a MIN and MAX calculation where no value is found. So cannot change that they return null.

Is there any other solution except the if?

I also have a table which specifies in which column the row can be found.

Is it possible to take the value from 1 out of 3 columns based on that value?

Yes you could use CASE to handle that.

$sql = "SELECT CASE table1.col1 
WHEN 'col1' THEN table2.col1 
WHEN 'col2' THEN table2.col2 
ELSE table2.col3 END";

Something like that should do it.

select coalesce(column1,column2,column3) ...

Yep. always forget about coalesce. 🙁