You also can:
- Select a field, right-click and select "Set NULL"
- Select a field, press CTRL + ALT + N (on Windows).
Note that the logical fields will be displayed as a field filled with a dash (-) when zero.
If the "Set NULL" option is not displayed, you may need to update the scheme, which can be done by clicking the synchronize button:

or by pressing Ctrl + Alt + Y (on Windows).
source share