Context :

Aims : We present the results of an analysis for 107 extremely metal-poor ( EMP ) stars with metallicities less than [ Fe/H ] = -3.0 , identified from medium-resolution spectra in the Sloan Digital Sky Survey ( SDSS ) .
Our analysis provides estimates of the stellar effective temperatures and surface gravities , as well as iron , calcium , and magnesium abundances .

Methods : We follow the same methodology as in previous papers of this series , based on comparisons of the observed spectra with synthetic spectra .
The abundances of Fe , Ca , and Mg are determined by fitting spectral regions dominated by lines of each element .
In addition , we present a technique to determine upper limits for elements whose features are not detected in a given spectrum .
We also analyse our sample with the SEGUE Stellar Parameter Pipeline , in order to obtain additional determinations of the atmospheric parameters , iron and alpha-element abundances , to compare with ours , and to infer [ C/Fe ] ratios .

Results : We find that , in these moderate to low signal-to-noise and medium-resolution spectra in this metallicity regime , Ca is usually the only element that exhibits lines that are sufficiently strong to reliably measure its abundance .
Fe and Mg exhibit weaker features that , in most cases , only provide upper limits .
We measure [ Ca/Fe ] and [ Mg/Fe ] for EMP stars in the SDSS spectra , and conclude that most of the stars exhibit the usual level of enhancement for \alpha -elements , \sim +0.4 , although a number of stars for which only [ Fe/H ] upper limits could be estimated point to higher [ \alpha /Fe ] ratios .
We also find that 26 % of the stars in our sample can be classified as carbon-enhanced metal-poor ( CEMP ) stars , and that the frequency of CEMP stars also increases with decreasing metallicity , as has been reported for previous samples .
We identity a rare , bright ( g = 11.90 ) EMP star , SDSS J134144.61 + 474128.6 , with [ Fe/H ] = -3.27 , [ C/Fe ] = +0.95 , and elevated magnesium ( [ Mg/Fe ] = +0.62 ) , an abundance pattern typical of CEMP-no stars .

Conclusions :