We have measured the CH G band ( CH ( G ) ) index for evolved stars in the globular cluster M3 based on the SDSS spectroscopic survey .
It is found that there is a useful way to select red giant branch ( RGB ) stars from the contamination of other evolved stars , such as asymptotic giant branch ( AGB ) and red horizontal branch ( RHB ) stars , by using the CH ( G ) index versus ( g - r ) _ { 0 } diagram if the metallicity is known from the spectra .
When this diagram is applied to field giant stars with similar metallicity we establish a calibration of CH ( G ) = 1.625 ( g - r ) _ { 0 } -1.174 ( g - r ) _ { 0 } ^ { 2 } -0.934 .

This method is confirmed by stars with \mbox { [ Fe / H ] } \sim - 2.3 where spectra of member stars in globular clusters M15 and M92 are available in the SDSS database .
We thus extend this kind of calibration to every individual metallicity bin ranging from \mbox { [ Fe / H ] } \sim - 3.0 to \mbox { [ Fe / H ] } \sim 0.0 by using field red giant stars with 0.4 \leq ( g - r ) _ { 0 } \leq 1.0 .
The metallicity-dependent calibrations give , CH ( G ) = 1.625 ( g - r ) _ { 0 } -1.174 ( g - r ) _ { 0 } ^ { 2 } +0.060 \mbox { [ Fe / H ] } -0.830 for -3.0 < \mbox { [ Fe / H ] } \leq - 1.2 and CH ( G ) = 0.953 ( g - r ) _ { 0 } -0.655 ( g - r ) _ { 0 } ^ { 2 } +0.060 \mbox { [ Fe / H ] } -0.650 for -1.2 < \mbox { [ Fe / H ] } < 0.0 .
The calibrations are valid for the SDSS spectroscopic data set , and they can not be applied blindly to other data sets .
With the two calibrations , a significant number of the contaminating stars ( AGB and RHB stars ) , were excluded and thus a clear sample of red giant stars is obtained by selecting stars within \pm 0.05 mag of the calibration .
The sample is published online and it is expected that this large and clean sample of RGB stars will provide new information on the formation and evolution of the Galaxy .