Gmail has some advanced searching capabilities that, if you take the time to learn, enable you drill down to very specific information. In this tip, you'll learn how to search for specific messages based on their Labels including how to search for multiple Labels.
If you want to search for all messages having a specific label, you can click on the "Show search options" link, click the "Search" dropdown, select the desired Label, and click the "Search Mail" button.
But a shortcut is to type the Label prefixed with the "label:" query word in any simple search field at the top of any Gmail page, like this:
If you want to view all messages that have selected multiple Labels, for example messages having both 'Label1' and 'Label2', enter the following into the simple search field at the top of any Gmail page.
To see all messages with either 'Label1' or 'Label2', you can enter:
Note: the specific label names are NOT case sensitive, but the "OR" operator is case sensitive, and must be in uppercase. The pipe operator '|' can also be used in the same manner as 'OR'.