select f.drinker
from   Frequents f join Bars b on f.bar = b.name
where  b.addr = 'Newtown';

Strategy 1:

Tmp1(drinker,bar,name,addr) = Frequents join[bar=name] Bars 
Result(drinker) = Sel[addr=Newtown](Tmp1)


Strategy 2:

NewtownBars(nbar) = Sel[addr=Newtown](Bars)
Tmp = Frequents Join[bar=nbar] NewtownBars
Result(drinker) = Proj[drinker](Tmp)