How to tell if ionic compound has a higher melting point?

[sgstudent]

I know that MgO has a higher melting point than NaCl as Mg is Mg2+ while O is O2- while Na is Na+ and Cl ia Cl-. but in CuCO3 and Na2CO3, Na2CO3 has a higher melting point compared to CuCO3. So I'm not sure how to tell which ionic compound will have a higher melting point than another when they are more complicated. Any guides on how to roughly tell this would greatly be appreciated.

Thanks so much for the help!

[Borek]

Simple approaches won't work for more complicated compounds.

[sgstudent]

Oh ok thanks this needs lattice energy and other complicated stuff...