Table: retina_specs
Columns: model
ppi_pixels_per_inch
ppcm_pixels_per_cm
resolution
typical_viewing_distance_in_cm
pixels_per_degree_ppd

Question: How many different PPI does the model with ppcm of 87 have?
SQL: SELECT COUNT("ppi_pixels_per_inch") FROM retina_specs WHERE "ppcm_pixels_per_cm" = 87

Question: What's the resolution of the model with a PPD of 69?
SQL: SELECT "resolution" FROM retina_specs WHERE "pixels_per_degree_ppd" = 69