Large photometric surveys are producing, and will continue doing it, massive amounts of data on small bodies. Usually,these data will be sparsely obtained at arbitrary (and unknown) rotational phases. Therefore, new methods to process such data need to be developed to make the most of those large catalogs. We aim to produce a method to create phase curves of small bodies considering the uncertainties introduced not only by the nominal errors in the magnitudes, but also the effect introduced by rotational variations.We use as a benchmark the data from the SLOAN Moving Objects Catalog with the objective to construct phase curves of all small bodies in there, in the u, g, r, i, and z, filters. We will obtain from the phase curves the absolute magnitudes and set up with them the absolute colors, which are the colors of the asteroids not affected by changes in phase angle. We select objects with >3 observations taken in, at least, one filter and spanned over a minimum of 5 degrees in phase angle. We developed a method that combines Monte Carlo simulations and Bayesian inference to estimate the absolute magnitudes using the HG12 photometric system. We obtained almost 15000 phase curves, about 12000 including all five filters. The absolute magnitudes and absolute colors are compatible with previously published data, supporting our method.Conclusions. The method we developed is fully automatic and well suited to be run on large amounts of data. Moreover, it includes the nominal uncertainties in the magnitudes and the whole distribution of possible rotational states of the objects producing, possibly,less precise values, i.e., larger uncertainties, but more accurate, i.e., closer to the real value. To the best of our knowledge, this work is the first to include the effect of rotational variations in such a way.