How to search by tags in org-mode

Question

Here is a simple file:

#+TAGS: math(m) music(u)

* Gödel :math:
* Bach :music:
* Gauss :math:
* Beethoven :music:
* Mozart :music:

Then I tried M-x org-tags-view

Match: +math

I expected that Gödel and Gauss will be printed, but instead I get an empty buffer that says:

Headlines with TAGS match: +math
Press ‘C-u r’ to search again with new search string

What's wrong?

Answer 1

org-tags-view shows matches in all agenda files: you have to add this file to your agenda files (at least temporarily). You can do that with C-c[: that adds it to the front of the list of agenda files (i.e. to the variable org-agenda-files). You can later delete it from the list if you want with C-c] (i.e., this will remove the file you're visiting from org-agenda-files). But while it's on the list, you can search for tags with org-tags-view.