As Eduard mentioned there are a couple of different approaches, depending on what your needs are.  Here are some simple classes to use as examples:

• PetOwner has a name, state and a list of Pets
• Pets has two properties: type and size

Class Demo.PetOwner extends %Persistent 
{
    Property Name As %String;
    Property State As %String;
    Property Pets As list of Pet;
}

Class Demo.Pet extends %SerialObject 

{
    Property Type As %String;
    Property Size As %String;
}

Running a simple query produces results with embedded characters and no easy way to parse the list of Pets:

SELECT Name, State, Pets FROM Demo.PetOwner

One option is to use a stored function to return a single value when querying a specific PetOwner:

/// SQL Function to get pet info
/// Return each pet info as a comma delimited string where type and size 
/// are separated by ^
ClassMethod PetInfo(id As %String = "") As %String [SqlProc] 
{
     Try {
         Set sc = $$$OK
         Set petInfo = ""
         // Quit if no id
         Quit:(id="")

         // Open the PetOwner
         #dim owner As Demo.PetOwner
         Set owner = ##class(Demo.PetOwner).%OpenId(id)

         #dim pet As Demo.Pet
         Set petCount = owner.Pets.Count()

         For i=1:1:petCount {
             Set pet = owner.Pets.GetAt(i)
             Set petInfo = petInfo _ pet.Type _ "^" _ pet.Size _ ","
         }

         // Remove trailing ,
         Set petInfo = $Extract(petInfo,1,*-1)
     }
     Catch(ex) {
         Set sc = ex.AsStatus()
     }

     Return petInfo
}

If PetInfo method is in the Demo.Test class, the stored function can be used like this to get a comma-delimited string of pet information where each component of the pet information is separated by a "^":

SELECT  Name, State, Demo.Test_PetInfo(ID) from Demo.PetOwner

Another option is to write a custom class query that is called as a stored procedure to return the results as a resultset:

CALL Demo.Test_GetOwnersWithPetDetails()

The GetOwnersWithPetDetails method is in the class Demo.Test and information about how to write a custom class query can be found in the Caché documentation.

Class Demo.Test extends %RegisteredObject 
{
    /// Create a custom query to handle getting all pet info
    /​// Assume maximum of three pets per PetOwner 
    Query GetOwnersWithPetDetails() As %Query(CONTAINID=1,ROWSPEC="ID:%Integer,Name:%String,State:%String,PetType1:%String,PetSize1:%String,PetType2:%String,PetSize2:%String,PetType3:%String,PetSize3:%String") [SqlProc] 
    {
    }

    /// Query initialization code goes here
    ClassMethod GetOwnersWithPetDetailsExecute(ByRef qHandle As %Binary) As %Status
    {
        Try {
            Set sc = $$$OK

            // Use a cursor to a query of the PetOwner table to the fetch method
            // The cursor is a public variable!
            // Declare the cursor
            &sql(DECLARE DemoC1 CURSOR FOR
                 SELECT ID FROM Demo.PetOwner ORDER BY ID)
  
            // Open the cursor
            &sql(OPEN DemoC1)

            Set qHandle=""
        } 
        Catch (ex) {
            Set sc = ex.AsStatus()
        }
        Return sc
    }

    ClassMethod GetOwnersWithPetDetailsFetch(ByRef qHandle As %Binary, ByRef Row As %List, ByRef AtEnd As %Integer=0) As %Status [PlaceAfter = GetOwnersWithPetDetailsExecute ]
    {
        Try {
            Set sc = $$$OK

            &sql(FETCH DemoC1 INTO :ownerID)

            // Assume maximum of three pets per PetOwner
            If (SQLCODE = 0) {
                // Initialize pet variables in case some are null
                Set PetType1 = ""
                Set PetSize1 = ""
                Set PetType2 = ""
                Set PetSize2 = ""
                Set PetType3 = ""
                Set PetSize3 = ""

                // Open the PetOwner with id from the cursor
                #dim owner As Demo.PetOwner
                Set owner = ##class(Demo.PetOwner).%OpenId(ownerID)
                Set Name = owner.Name
                Set State = owner.State

                Set numPets = owner.Pets.Count()
                #dim pet As Demo.Pet
                If (numPets>=1) {
                    Set pet = owner.Pets.GetAt(1)
                    Set PetType1 = pet.Type
                    Set PetSize1 = pet.Size
                }

                If (numPets>=2) {
                    Set pet = owner.Pets.GetAt(2)
                    Set PetType2 = pet.Type
                    Set PetSize2 = pet.Size
                }

                If (numPets=3) {
                    Set pet = owner.Pets.GetAt(3)
                    Set PetType3 = pet.Type
                    Set PetSize3 = pet.Size
                }
  
                Set Row = $ListBuild(ownerID,Name,State,PetType1,PetSize1,PetType2,PetSize2,PetType3,PetSize3)
                Set AtEnd = 0
            }
            ElseIf (SQLCODE = 100) {
                Set Row = ""
                Set AtEnd = 1
            }
            Else {
                Set sc = $System.Status.Error(5001,"SQL fetch error: "_SQLCODE)
                Quit
            }
        } 
        Catch (ex) {
            Set sc = ex.AsStatus()
        }
        Return sc
    }

   ClassMethod GetOwnersWithPetDetailsClose(ByRef qHandle As %Binary) As %Status [PlaceAfter = GetOwnersWithPetDetailsExecute ]
   {
       Try {
           Set sc = $$$OK
           &sql(CLOSE DemoC1)
       } 
       Catch (ex) {
           Set sc = ex.AsStatus()
       }
       Return sc
   }
}