Clear explanation of the “theta join” in relational algebra?

Question

I\'m looking for a clear, basic explanation of the concept of theta join in relational algebra and perhaps an example (using SQL perhaps) to illustrate its usage.

If I

Answer 1

In my opinion, to make it simple, if you understand equijoin, you suppose should understand theta join. If you change the symbol = (equal) in equijoin to >=, then you already done theta join. However, I think it is quite difficult to see the practicality of using theta join as compared to equijoin since the join cause that we normally use is V.primarykey = C.foreignkey. And if you want to change to theta join, then it may depends on the value, as such you are doing selection.

For natural Join, it is just similar to equijoin, the difference is just that get rid of the redundant attributes. easy!:)

Hope this explanation helps.