By now I know how to import "SQL-Data" into a sheet.
But I cannot append the "SQL-Data" to an existing sheet, which is meant to grow over time.
//Pseudo Code
1. maxID <-- getMaxValueOutOfColumnId()
2. import via SQL --> "SELECT * FROM table WHERE ID > maxID"
3. append the result set as rows to the sheet maxID came from.
I have found tutorials on importing stuff but they did not append SQL-Data.
For future visitors I post the solution I came up with.