Can i dynamically create a sql query statement in a method?

Yes the approach you are saying is correct.The correct approach of creating data access layer is to create a common class
that should have the methods for fetching as well as updating the database.
Following steps should help you implement the approach you have said above

1) Create a data access class having the methods for fetching ,updating and deleting records.These should be common methods. 
It is better if you create stored procedures corresponding to these methods.

2) Create the entity classes representing the entities or tables in the database.This class should have properties you have mentioned above.
3) Create an instance of the data access class in the step one above.Create the instances of the entity class in the step 2 above and pass the instance of the entity class to the methods of the data access class.You can also assign the result of the data access class method to an entity class instance if you are fetching values.