Contents

.TH Intersect
.PP
Logical AND
.SH Signature
.PP
.RS
.nf
intersect(left: Relation, right: Relation) \-> Relation
.fi
.RE
.SH Examples
.PP
.RS
.nf
intersect(
  restrict(suppliers, eq(:city, 'Paris')),
  restrict(suppliers, gt(:status, 10)))
.fi
.RE
.SH Description
.PP
Computes the relation as the set intersection of \fB\fCleft\fR and \fB\fCright\fR\&.
.PP
The \fB\fCleft\fR and \fB\fCright\fR relations must be intersect\-compatible, meaning that
they must have same heading (type inheritance is partly supported through
ruby's own type system, so that the actual behavior is slighlty more
permissive).
.SH Implementation notes
.PP
Unlike SQL, this operator ALWAYS remove duplicates. There is no way, in
Alf, to compute \fIbags\fP of tuples and therefore no way to express something
such as SQL's INTERSECT ALL.
.PP
It is sometimes idiomatic in Alf to use \fB\fCintersect\fR as a logical AND, as
illustrated below. So far, the optimizer/compiler is not smart enough to
translate the former into the latter (which is likely to have a better query
when using faithful SQL compilation and available SQL DBMSs). Any patch is
welcome here too!
.PP
.RS
.nf
intersect(
  restrict(suppliers, eq(:city, 'Paris')),
  restrict(suppliers, gt(:status, 10)))
.fi
.RE
.PP
is equivalent to
.PP
.RS
.nf
restrict(suppliers, eq(:city, 'Paris') & gt(:status, 10))
.fi
.RE

Version data entries

3 entries across 3 versions & 1 rubygems

Version	Path
alf-shell-0.16.3	doc/man/intersect.man
alf-shell-0.16.2	doc/man/intersect.man
alf-shell-0.16.1	doc/man/intersect.man