Could you explain what is the usage of Modules with select query?
For example if I write (as shown on this page https://cumulocity.com/guides/users-guide/administration/):
select * from MeasurementCreated
Is it useful to get real time notifications by subscribing of the related channel? Is the module reachable by an angularJs Module? Can this module be used in other CEL statements?
Just selecting data without putting it into another stream can make sense in the case you want to make this data available via a real-time channel to some external application (this could be of course AngularJs).
Take a look at this section in the docs: http://cumulocity.com/guides/reference/real-time-statements/#notifications
This very one example though does not make a lot of sense because raw measurement data is already provided on a real-time channel http://www.cumulocity.com/guides/reference/measurements/#notifications
As for the second part of the question: Yes it is possible to communicate with other modules within your tenant. e.g. You can declare some stream in module a and it will be available in module b.
