  We present a catalog of 1,172,157 quasar candidates selected from the
photometric imaging data of the Sloan Digital Sky Survey (SDSS). The objects
are all point sources to a limiting magnitude of i=21.3 from 8417 sq. deg. of
imaging from SDSS Data Release 6 (DR6). This sample extends our previous
catalog by using the latest SDSS public release data and probing both UV-excess
and high-redshift quasars. While the addition of high-redshift candidates
reduces the overall efficiency (quasars:quasar candidates) of the catalog to
~80%, it is expected to contain no fewer than 850,000 bona fide quasars -- ~8
times the number of our previous sample, and ~10 times the size of the largest
spectroscopic quasar catalog. Cross-matching between our photometric catalog
and spectroscopic quasar catalogs from both the SDSS and 2dF Surveys, yields
88,879 spectroscopically confirmed quasars. For judicious selection of the most
robust UV-excess sources (~500,000 objects in all), the efficiency is nearly
97% -- more than sufficient for detailed statistical analyses. The catalog's
completeness to type 1 (broad-line) quasars is expected to be no worse than
70%, with most missing objects occurring at z<0.7 and 2.5<z<3.0. In addition to
classification information, we provide photometric redshift estimates
(typically good to Delta z +/- 0.3 [2 sigma]) and cross-matching with radio,
X-ray, and proper motion catalogs. Finally, we consider the catalog's utility
for determining the optical luminosity function of quasars and are able to
confirm the flattening of the bright-end slope of the quasar luminosity
function at z~4 as compared to z~2.
