  We assemble a large sample of 12,784 high-velocity stars with total velocity
$\it{V}_{\rm{GSR}}\ge {\rm 300}$ km s$^{-1}$, selected from RAVE DR5, SDSS
DR12, LAMOST DR8, APOGEE DR16, GALAH DR2, and $Gaia$ EDR3. In this sample, 52
are marginally hypervelocity star (HVS) candidates that have
$\it{V}_{\rm{GSR}}$ exceeding their local escape velocities within $2\sigma$
confidence levels, 40 of which are discovered for the first time. All
candidates are metal-poor, late-type halo stars, significantly different from
the previous identified HVSs, which are largely massive early-type stars,
discovered by extreme radial velocity. This finding suggests that our newly
identified HVS candidates are ejected by different mechanisms from the previous
population. To investigate their origins, for 547 extreme velocity stars with
${V}_{\rm{GSR}}\ge0.8V_{\rm{esc}}$, we reconstruct their backward-integrated
trajectories in the Galactic potential. According to the orbital analysis, no
candidates are found to be definitely ejected from the Galactic-center (GC),
while 8 metal-poor extreme velocity stars are found to have a closest distance
to the GC within 1kpc. Intriguingly, 15 extreme velocity stars (including 2 HVS
candidates) are found to have experienced close encounters with the Sagittarius
dSph, suggesting that they originated from this dSph. This hypothesis is
supported by an analysis of the [$\alpha$/Fe]--[Fe/H] diagram. From a
preliminary analysis of all the 547 extreme velocity stars, we propose a
general picture: Star ejection from Galactic subsystems such as dwarf galaxies
and globular clusters can be an important channel to produce extreme velocity
stars or even HVSs, particularly the metal-poor late-type halo population.
