My SQL - GROUP BY

[Jezevec]

select COUNT(*), COUNT(*), m.datum 

from Mereni m, Mereni n 

where m.idTyp =5 
and n.idTyp = 5 

and n.datum = m.datum 

and m.intuns_b = 128 
and n.intuns_b=128 

and m.intuns_c = 84 
and n.intuns_c=84 

and m.intuns_e = 2 
and n.intuns_e=2 

and m.platne = 1 
and n.platne=1 

and m.hodnota > 120 

GROUP BY m.datum

a) count(*) lze pokud vim pouzit jen jednou a je to pocet vsech radku
b) nebylo by od veci se naucit pouzivat join
c) ty joinujes tabulku samu na sebe ?? duvod ?
d) kdyz uz neco joinujes, tak do dusledku

pr:

from table_a
join table_b on table_a.x = table_b.x and table_a.y = table_b.y ...

Resil bych nejspi subselectem pr:

select count(*) / (select count(*) from mereni) as podil, 
         datum
from mereni
where ...