About
A theta-join is a difficult/complex join where the condition is not a equality.
Example:
- Band join
- or range join.
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).
Notation
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.
Example
Find all customer in a age range
Find all the hospitals within 5 miles of a school
select distinct h.name
from hospitals h, schools s
where
distance(h.location,s.location) < 5
where: Distance
Find all user clicks made within 5 seconds of page load
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