There is no possibility to read data from an external database with standard c/al. Best way to do that till NAV version 2009 is using ADO. With Nav vs. 2013 upwards you can use .Net classes.
The following code can be used to read data from a given, external database.

// local variablesADOConnection  Automation'Microsoft ActiveX Data Objects 2.8 Library'.ConnectionADOCommand  Automation'Microsoft ActiveX Data Objects 2.8 Library'.CommandADOParameter  Automation'Microsoft ActiveX Data Objects 2.8 Library'.ParameterADORecSet  Automation'Microsoft ActiveX Data Objects 2.8 Library'.RecordsetadoField  Automation'Microsoft ActiveX Data Objects 2.8 Library'.FieldconnString  Text1024activeConnection  VariantsalesLineStr  Text1024idx  IntegeradoValue  VariantadoType  Integer// the code// replace the server ip and the ext. databasename with your valuesReadFromExternalDatabase()connString := 'Driver={SQL Server};Server=127.0.0.1;Database=Cronus600';CREATE(ADOConnection);ADOConnection.ConnectionString(connString);ADOConnection.Open;// convert ADO Connection to VariantactiveConnection := ADOConnection;CREATE(ADOCommand);ADOCommand.ActiveConnection := activeConnection;ADOCommand.CommandType := 1; // type is sql statement// sample statement: read first record from table sales lineADOCommand.CommandText := 'select top 1 * from [CRONUS AG$Sales Line]';ADOCommand.CommandTimeout := 100;CREATE(ADORecSet);ADORecSet := ADOCommand.Execute;ADORecSet.MoveFirst;  // goto first recordFOR idx := 1 TO ADORecSet.Fields.Count - 1 DO BEGIN  adoField := ADORecSet.Fields.Item(idx);  adoType := adoField.Type;  IF adoType  131 THEN  // ado type numeric must be convertedadoValue := adoField.Value  ELSEadoValue := GetFieldValue(adoField);  IF (adoField.Type  13) THEN  // ado type 13 = Unknown// gets the value with index idx of the resulting recordsalesLineStr := salesLineStr + FORMAT(adoValue);  salesLineStr := salesLineStr + ';';END;MESSAGE(salesLineStr);ADORecSet.Close;ADOConnection.Close;CLEARALL;GetFieldValue(adoField : 'Microsoft ActiveX Data Objects 2.8 Library'.Field) : Text// local variables// ADOStream  Automation'Microsoft ActiveX Data Objects 2.8 Library'.Stream// RetVal  TextCREATE(ADOStream);ADOStream.Open;ADOStream.WriteText(adoField.Value);ADOStream.Position:= 0;RetVal:= ADOStream.ReadText;ADOStream.Close;exit(RetVal);

helpful links:
http://www.w3schools.com/asp/ado_ref_connection.asp
http://www.w3schools.com/asp/ado_ref_command.asp
http://www.w3schools.com/asp/prop_comm_commandtype.asp#commandtypeenum