  We have derived element abundances in 310 emission-line galaxies from the
Early Data Release of the Sloan Digital Sky Survey (SDSS) for which the [O {\sc
iii}] 4363 emission line was detected, allowing abundance determination by
direct methods. We found no extremely metal-deficient galaxy (Z<Zsun/12),
probably as a consequence of selection effects in the SDSS sample. The oxygen
abundance 12 + log O/H of the SDSS galaxies sample lies in the range from ~7.6
(Zsun/12) to ~8.4 (Zsun/2). This sample is merged with a sample of ~100 blue
compact dwarf galaxies with high quality spectra containing some very
low-metallicity objects to study the abundance patterns of low-metallicity
emission-line galaxies. We find that the $\alpha$ element-to-oxygen abundance
ratios do not show any significant trends with the oxygen abundance, in
agreement with previous studies. The Fe/O abundance ratio is smaller than the
solar value, which we interpret as an indication that type Ia supernovae have
not yet appeared in these galaxies, implying an age of less than 1-2 Gyr.
However, a slight decrease of the Fe/O abundance ratio with increasing
metallicity suggests some depletion of iron onto dust in the galaxies with
higher metallicities. The N/O abundance ratio ranges from log N/O= -1.6 to
-0.8. The fact that no galaxy with log N/O < -1.6 was discovered implies that
local low-metallicity emission-line galaxies are of a different nature than
high-redshift damped Lyalpha systems with log N/O of ~-2.3 and that their ages
are probably larger than 100-300 Myr. Our data indicate the existence of a
gradual nitrogen enrichment on a time-scale of a few Myr.
