Sodium nitrate decomposes above 800° C to give

Sodium nitrate on decomposition upto 500°C
gives NaNO₂ and oxygen.
$2{\mathrm{NaNO}}_3\stackrel{\mathrm{\Delta }}{⟶}2{\mathrm{NaNO}}_2+{\mathrm{O}}_2\uparrow$
While at higher temperature (i.e. above to 800°C),${\mathrm{NaNO}}_2\text{ further decomposes into }{\mathrm{Na}}_2\mathrm{O},{\mathrm{N}}_2\text{ and }{\mathrm{O}}_2$.
$2{\mathrm{NaNO}}_2\stackrel{{800}^{\circ }\mathrm{C}}{⟶}{\mathrm{Na}}_2\mathrm{O}+3/2{\mathrm{O}}_2\uparrow +{\mathrm{N}}_2\uparrow$