  We discuss the characteristics of known extremely metal-poor (EMP) stars in
the Galaxy using the Stellar Abundances for Galactic Archaeology (SAGA)
database (Suda et al. 2008, PASJ, 60, 1159).The analyses of carbon-enhanced
stars in our sample suggest that the nucleosynthesis in AGB stars can
contribute to the carbon enrichment in a different way depending on whether the
metallicity is above or below [Fe/H] ~ -2.5, which is consistent with the
current models of stellar evolution at low metallicity. We find the transition
of the initial mass function at [Fe/H] ~ -2 in the viewpoint of the
distribution of carbon abundance and the frequency of carbon-enhanced stars.
For observed EMP stars, we confirmed that some, not all, of observed stars
might have undergone at least two types of extra mixing to change their surface
abundances. One is to deplete the lithium abundance during the early phase of
red giant branch. Another is to decrease the C/N ratio by one order of
magnitude during the red giant branch phase. Observed small scatters of
abundances for alpha-elements and iron-group elements suggest that the chemical
enrichment of our Galaxy takes place in a well-mixed interstellar medium. We
find that the abundance trends of alpha-elements are highly correlated with
each other, while the abundances of iron-group elements are subject to
different slopes relative to the iron abundance. This implies that the
supernova yields of alpha-elements are almost independent of mass and
metallicity, while those of iron-group elements have a metallicity dependence
or mass dependence with the variable initial mass function.The occurrence of
the hot bottom burning in the mass range of 5 <~ M / Msun <~ 6 is consistent
with the initial mass function of the Galaxy peaked at ~ 10 - 12 Msun to be
compatible with the statistics of carbon enhanced stars with and without
s-process element (truncated)
