[[AgustinOrtu]]

Creo que la mejor forma de independizar del motor es usando interfaces:

Código Delphi [-]
unit SQLFunctions;

interface

uses
  Data.DB;

type
  ISQLFunctions = interface
    ['{CD84579F-2EE3-4842-BCF3-F8B5A2B7C50B}']
    function NullFx(const ParamName, FieldName: string): string;
  end;

  TExtendedSQLConnection = class abstract(TInterfacedObject, ISQLFunctions)
  protected
    FConnection: TCustomConnection;
    function NullFx(const ParamName, FieldName: string): string; virtual; abstract;
  public
    constructor Create(const AConnection: TCustomConnection); virtual;
  end;

implementation

uses
  System.SysUtils;

{ TExtendedSQLConnection }

constructor TExtendedSQLConnection.Create(const AConnection: TCustomConnection);
begin
  if not Assigned(AConnection) then
    raise Exception.Create('TExtendedSQLConnection.Create :: AConnection is not Assigned');

  inherited Create;
  FConnection := AConnection;
end;

end.

Código Delphi [-]
unit MySQLFunctions;

interface

uses
  SQLFunctions;

type
   TMySQLExtendedConnection = class(TExtendedSQLConnection)
   protected
    function NullFx(const ParamName, FieldName: string): string; override;
   end;

implementation

uses
  System.SysUtils;

{ TMySQLExtendedConnection }

function TMySQLExtendedConnection.NullFx(const ParamName, FieldName: string): string;
begin
  Result := Format('IfNull(:%s, %s)', [ParamName, FieldName]);
end;

end.

Código Delphi [-]
unit MSSQLFunctions;

interface

uses
  SQLFunctions;

type
   TMSSQLExtendedConnection = class(TExtendedSQLConnection)
   protected
    function NullFx(const ParamName, FieldName: string): string; override;
   end;

implementation

uses
  System.SysUtils;

{ TMSSQLExtendedConnection }

function TMSSQLExtendedConnection.NullFx(const ParamName, FieldName: string): string;
begin
  Result := Format('IsNull(:%s, %s)', [ParamName, FieldName]);
end;

end.