Aggregation
A simple query that cannot be expressed in Relational Algebra, or in the
SQL we've seen so far:
How many rows are in table T?
Reviewing Relational Algebra operators:
- select: Keep or reject a row.
- project: Keep or reject a column.
- product: Concatenate rows.
- join: Match rows and possibly concatenate them.
- divide: Keep or reject a row based on set comparison.
- rename: Metadata operation.
- union, intersect, difference: Keep or reject rows.
What about SQL functions?
- Scalar operations so far: compute a value given
column values in one row.
So far, we don't have a way to compute functions across multiple rows.
|