Semi Joins (not in book)
Left semi-join of R1 and R2:
- Each row of R1 is including in the output (with nulls in R2 columns if necessary), even if there are no matches in R2 for that row.
Right semi-join:
- Each row of R2 is included, etc.
Quiz: suppose R1 and R2 have no matches in their common columns. What is the cardinality of: Cartesian product; natural join; left, right semi-joins?