  Based on 5344 quasar spectra taken from the SDSS Data Release 2, the
dependences of various emission-line flux ratios on redshift and quasar
luminosity are investigated in the ranges 2.0 < z < 4.5 and -24.5 > M_B >
-29.5$. We show that the emission lines in the composite spectra are fitted
better with power-law profiles than with double Gaussian or modified Lorentzian
profiles, and in particular we show that the power-law profiles are more
appropriate to measure broad emission-line fluxes than other methods. The
composite spectra show that there are statistically significant correlations
between quasar luminosity and various emission-line flux ratios, such as NV/CIV
and NV/HeII, while there are only marginal correlations between quasar redshift
and emission-line flux ratios. We obtain detailed photoionization models to
interpret the observed line ratios. The correlation of line ratios with
luminosity is interpreted in terms of higher gas metallicity in more luminous
quasars. For a given quasar luminosity, there is no metallicity evolution for
the redshift range 2.0 < z < 4.5. The typical metallicity of BLR gas clouds is
estimated to be Z ~ 5 Z_sun, although the inferred metallicity depends on the
assumed BLR cloud properties, such as their density distribution function and
their radial distribution. The absence of a metallicity evolution up to z ~ 4.5
implies that the active star-formation epoch of quasar host galaxies occurred
at z > 7.
