Theta JOIN (Relational Operation)

The general case of the JOIN operation is called a Theta-join:

$$R\underset{\text{theta}}{\bowtie }S$$

The join condition is called theta. Theta can be any general boolean expression on the attributes of $R$ and $S$, e.g.:

$$R.Ai<S.Bj\text{ and }(R.Ak=S.Bi\text{ or }R.Ap<S.Bq)$$