Relational matching (join + project back on left)
SYNOPSIS
#(signature)
DESCRIPTION
This operator restricts its LEFT operand to tuples for which there exists at least one tuple in RIGHT that (naturally) joins. This is a shortcut operator for the following longer expression:
(project (join xxx, yyy), [xxx's attributes])
Or equivalently,
(join xxx, (project xxx, [common attributes]))
EXAMPLE
# Which suppliers supply at least one part?
!(alf matching suppliers supplies)