Mysql query is used for "handwritten" queries. You can write your own query in the Query field.
If your query type is SELECT - then you need to fill in Select Store To fields.
Important - if you want Store To String - select Store To String variable and leave Store To Number (none) (case 1). If you want Store To Number - select Store To Number and leave (none) to Store To String (case 2). If you use INSERT or other Query type - leave this fields (none) (case3)