1 INTRODUCTION  
2 DATA AND DERIVED QUANTITIES  
2.1 SDSS sample $(z\sim\!0)$  
2.2 The KLEVER survey $(z\sim\!2.2)$  
2.3 Star formation rates  
2.4 Metallicity determination  
2.5 Nitrogen abundances  
3 RESULTS AND DISCUSSION  
3.1 The relationship between O/H and N/O  
3.2 N/O and stellar mass  
3.3 The Fundamental Metallicity Relation (FMR)  
3.4 A Fundamental Nitrogen Relation (FNR)?  
4 CONCLUSIONS