Re:We Can Do Better Than SQL
by cmrdporcupine on Tuesday August 11, 2020 @11:50AM
(#24121681)
"Selection" in the relational algebra is actually everything that comes after the WHERE clause. SQL confuses people here because "SELECT" seems to imply "select these columns" when in fact that is technically called "projection."
The "Selection" is the set of predicates that restrict the resulting relation. Projection is choosing which tuples ("columns") to use in it.
The relational algebra has no "tables", this is, again, a SQL thing. It has relations (sets of sets) and operations on them. In SQL "tables" are one kind of relation, and views are another.
The "Selection" is the set of predicates that restrict the resulting relation. Projection is choosing which tuples ("columns") to use in it.
The relational algebra has no "tables", this is, again, a SQL thing. It has relations (sets of sets) and operations on them. In SQL "tables" are one kind of relation, and views are another.