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In order to study the Milky Way, RR Lyrae (RRL) variable stars identified byGaia, ASAS-SN and ZTF sky survey projects have been analyzed as tracers in thiswork. Photometric and spectroscopic information of 3417 RRLs including propermotions, radial velocity and metallcity are obtained from observational data ofGaia, LAMOST, GALAH, APOGEE and RAVE. Precise distances of RRLs with typicaluncertainties less than 3% are derived by using a recent comprehensiveperiod-luminosity-metallicity relation. Our results from kinematical andchemical analysis provide important clues for the assembly history of the MilkyWay, especially for the Gaia-Sausage ancient merger. The kinematical andchemical trends found in this work are consistent with that of recentsimulations which indicated that the Gaia-Sausage merger is the dual origin ofthe Galactic thick disc and halo. As recent similar works have found, the haloRRLs sample in this work contains a subset of radially biased orbits besides amore isotropic component. This higher orbital anisotropy component amounts to$\beta\simeq 0.8$, and it contributes between 42% and 83% of the halo RRLs at$4 < R(\rm kpc)<20$.

The Milky Way Revealed by Variable Stars I: Sample Selection of RR Lyrae stars, and Evidence for the Merger History