In queries, we use some expressions as follows:
Seniority: (Select Count(*) from Employees Where [HireDate]
[EmployHireDate].[HireDate]+1

which are very useful for our desired calculation.
These expressions are not meaningful for me, although I can copy-paste and
use them but I would like to be able to write them by my own. For example
what is the meaning of "Select Count(*)" in above expression?
My question is that where can I find the format of these expressions and a
basic tutorial about it?

Any comment is highly appreciated.
TIA, Rasoul Khoshravan Azar
Tabriz, Iran