To open a database object in a window:
Select the object in Project Navigator pane
Click Open button on the Project Navigator pane's toolbar.
When the IDEAl user interface mode (the default) is used, each window has it's own tab. Move the mouse pointer to the tab. You will see close button. Click it to close the tab.
In the Childframe on the right hand of each opened window there are buttons you can use to control the window. Click the first one on the right hand to close the window.
Alternatively, regardless of the user interface mode you are using, you can select Window ->Close from the Menubar.
Window's buttons
Other button's (from right to left) can be used to: maximize, minimize and undock the window.
There's a small icon on the left hand of the window which can be clicked to show a context menu with commands related to the window.