We present a method to determine sodium-abundance ratios ([Na/Fe]) using the Na I D doublet lines in low-resolution ($R sim$ 2000) stellar spectra. As stellar Na I D lines are blended with those produced by the interstellar medium (ISM), we developed a technique for removing the interstellar Na I D lines using the relationship between extinction, which is proportional to $E(B-V)$, and the equivalent width (EW) of the interstellar Na I D absorption lines. When measuring [Na/Fe], we also considered corrections for non-local thermodynamic equilibrium (NLTE) effects. Comparisons with data from high-resolution spectroscopic surveys suggest that the expected precision of [Na/Fe] from low-resolution spectra is better than 0.3 dex for stars with [Fe/H] $>$ $-$3.0. We also present a simple application employing the estimated [Na/Fe] values for a large number of stellar spectra from the Sloan Digital Sky Survey (SDSS). After classifying the SDSS stars into Na-normal, Na-high, and Na-extreme, we explore their relation to stars in Galactic globular clusters (GCs). We find that, while the Na-high SDSS stars exhibit a similar metallicity distribution function (MDF) to that of the GCs, indicating that the majority of such stars may have originated from GC debris, the MDF of the Na-normal SDSS stars follows that of typical disk and halo stars. As there is a high fraction of carbon-enhanced metal-poor stars among the Na-extreme stars, they may have a non-GC origin, perhaps due to mass-transfer events from evolved binary companions.
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