Behind the WHERE is a logical expression; these are similar to other languages as well, with operators AND, OR, and NOT.
SELECT source_id, ra, dec FROM gaiadr1.tgas_source WHERE phot_g_mean_flux > 13 AND parallax < 0.2
Select the absolute magnitude, position and the source id for the 20 stars with the greatest observed magnitude in the g band from the table tgas_source (in case you don’t remember: The absolute magnitude is M= 5 + 5log π+ m with the parallax in arcsec π and the apparent magnitude m (check the units!). This will fail. Try to fix it.