select min(placa_pod), max(placa_pod), max(placa_pod)-min(placa_pod) from pracownicy;
select avg(placa_pod), etat from pracownicy group by etat order by 1 desc;
select count(*) from pracownicy;
select sum(placa_pod + IFNULL(placa_dod,0)), id_zesp from pracownicy group by id_zesp;
select id_zesp, count(*) from pracownicy group by id_zesp having count(*) > 2 order by 2 desc;
select avg(placa_pod), count(*), etat from pracownicy where zatrudniony > '1990-01-01' group by etat;
select nazwisko, min(placa_pod) from pracownicy group by id_szefa sort by 2 desc;
select id_prac from pracownicy group by id_prac having count(*) > 1