Multiple Result Tabs in Workbench
You have actually 3 possibilities to run the queries:


Run everything that's in the editor. This is like running an external script. Workbench opens a result tab for each statement that returns a result set. Use the flash icon or cmd/ctrl+shift+enter to execute (cmd on macOS, otherwise ctrl).
Select a single statement or multiple statements and press the same shortcut as in point 1. This will only run the selected code. The editor is clever enough to find statement boundaries automatically to run complete statements.
Place your cursor in the statement you want to run and press cmd/ctrl+enter. This will only run this single statement (and hence only show a single result set).