保存流 出现错误：[FireDAC][Phys]-329. Cannot generate update query. WHERE condition is empty

procedure SaveData(const Key: Integer; Stream: TMemoryStream);
var db: TFDQuery;
begin
db := TFDQuery.Create(nil);

try
db.Connection := Connection;
db.SQL.Text   := 'SELECT [' + fnData + '] FROM [' + tbOutpatientCards + '] '+
'WHERE [' + fnKey + '] = ' + IntToStr(Key);
db.Open;

if db.RecordCount > 0 then
begin
db.Edit;
(db.FieldByName(fnData) as TBlobField).LoadFromStream(Stream);
db.Post;
end;
finally
db.Free;
end;