A theta-join is a difficult/complex join where the condition is not a equality.
Example:
A theta is a join that links tables based on a relationship other than the equality between two columns.
A theta join could use any other operator than the equal operator.
A theta may not have any join key in the sql but you still have a join physically (ie when running the sql).
Algebraic rule
Physically, the join of R1 and R2 with the condition theta is the same than the selection of the cross product of R1 and R2 with the condition theta.
select distinct h.name
from hospitals h, schools s
where
distance(h.location,s.location) < 5
where: What is a Distance?
Find all user clicks made within 5 seconds of page load
select *
from Clicks c, PageLoads p
where
abs(c.click_time - p.load_time) < 5