`p(x|z,y,t)`, untested

all_yzt.sql

select numerator.x, numerator.num/denominator.denom, numerator.y, numerator.z, numerator.t
(select sum(x) as num, x, y, z, t from table group by x, y, z, t) numerator
join
(select sum(x) as denom, y, z, t from table group by y,z,t) denominator
on (numerator.y=denominator.y and numerator.z=denominator.z and numerator.t=denominator.t)

query.sql

select numerator.x, numerator.num/denominator.denom
(select sum(x) as num, x from table where y=Y and z=Z and t=T group by x) numerator
cross join
(select sum(x) as denom from table where y=Y and z=Z and t=T) denominator