  A database of radio-quiet quasars detected with ROSAT is presented containing
846 quasars seen in the All-Sky Survey and/or in pointed PSPC observations.
About 70% of the objects have been detected in X-rays for the first time. We
present the soft X-ray fluxes and spectra, if available. Using an optically
selected subsample compiled from this database, we study the broad band
properties of radio-quiet quasars with high statistical significance. We
confirm that radio-quiet quasars have in general steeper soft X-ray spectra
than radio-loud. The spectral differences persist to high redshifts. A spectral
flattening with increasing redshift is confirmed up to redshift around 2,
beyond which the spectral slopes seem to be independent of redshift. The
spectral slopes of the ROSAT radio-quiet quasars at z>2.5 are consistent with
those found for nearby quasars in the medium energy band (2-10keV), implying
that the X-ray spectral evolution is not important. We show that there is, in a
statistical sense, little or no excess absorption for most of the radio-quiet
objects at z>2. The existence of a correlation between the X-ray luminosity and
the luminosity at 2500A is confirmed. Individual objects show a large scatter
from this correlation. The X-ray loudness alpha_ox appears to be independent of
z, but the analyses indicate a slight increase of alpha_ox with optical
luminosity. However, this behavior is likely not caused by physical properties
inherent to the quasars but is the result of the intrinsic dispersion. Finally,
we find a small fraction of objects which appear to be relatively `X-ray quiet'
compared to the bulk of the other quasars.
