Creating Queries and Custom Operations 
A query is a data retrieval operation that is modeled by applying detailed selection criteria; a custom operation is a service operation for validations, business logic, and that like that must be coded manually.
Prerequisites
You have created data objects to use in your query or custom operation.
More information: Creating the Data Model
Procedure
1. Create a Service
More information: Creating a Service
2. Model the Query or Custom Operation
You define the parameters for your query.
More information: Modeling a Query
You define the parameters for your custom operation and provide code for its implementation.
More information: Modeling a Custom Operation