How to find all TODOs that are scheduled today or before today or TODOs that are not scheduled

Question

How to find all TODOs in org-mode that meet any of the following conditions:

• TODOs that are scheduled to be done today
• TODOs that are scheduled to be done before today
• TODOs that are not scheduled

Answer 1

I figured it out. Press C-c a m and enter the following search criteria:

SCHEDULED=""|SCHEDULED<"<tomorrow>"/TODO

For more information, you can visit the official doc, I also find this tutorial extremely useful.