An example

The following example defines a Jelly View which gives information about siblings. Data about parents are given as extensional knowledge in the form of relation in the database. They are held by a relational schema: Parent(child_name,parent_name). Its state is given in Table 8.3.

Table 8.3: State of Parent(child_name,parent_name).

Parent:
child_name	parent_name
john	james
mary	james
ann	mary
simon	mary
michael	ann
alice	john

The Jelly View is generated by the following Prolog program:

  sibling(X,Y) :- parent(X,Z), parent(Y,Z), X \== Y.

This program can be presented, in a decomposed form, as a set of relations given in Table 8.4. The External Matching is given in Table 8.5. The Internal Matching is given in Table 8.6.

Table 8.4: Decomposed Prolog Program.

clause
id	name	order	preconditioned	symbol
1	sibling	NULL	NULL	1
2	parent	1	1	2
3	parent	2	1	2
4	$\backslash$==	3	1	4

argument
name	position	clause
X	1	1
Y	2	1
X	1	2
Z	2	2
Y	1	3
Z	2	3
X	1	4
Y	2	4

logical operator
id	symbol
1	:-
2	,
3	;
4	.

Table 8.5: External Matching.

table-predicate
id	predicate	table
1	sibling	sibling

tp-argument
name	type	table-predicate	position
name1	varchar	1	1
name2	varchar	1	2

Table 8.6: Internal Matching.

predicate-table
id	predicate	arity	table
1	parent	2	Parent

Table 8.7: Auxiliary Relations.

tp-clause
program	clause	order
1	1	10

tp-pt
table-predicate	predicate-table
1	1

The auxiliary relations are given in Figure 8.7. The relation tp-pt is used to establish a many-to-many relationship between table-predicate and predicate-table, which is denoted as has in Figure 8.3, while the relation tp-clause establishes many-to-many relationships between table-predicate and clause and it is denoted as consists_of in Figure 8.3. Description of the attributes for the Matchings and auxiliary relations is given in Table 8.8. Description for the program is analogous as the description for Table 8.1.

Table 8.8: Attributes of the Matchings and auxiliary relations.

attribute	description
table-predicate.id	primary key
table-predicate.predicate	goal name
table-predicate.table	Jelly View name
tp-argument.name	Jelly View argument name
tp-argument.type	Jelly View argument type in terms of RDBMS
tp-argument.table-predicate	foreign key referring to table-predicate.id
tp-argument.position	position of the argument, an arbitrary integer
predicate-table.id	primary key
predicate-table.predicate	predicate name
predicate-table.arity	predicate arity
predicate-table.table	relation name
tp-clause.program	foreign key referring to table-predicate.id
tp-clause.clause	foreign key referring to clause.id
tp-clause.order	clause order in the program
tp-pt.table-predicate	foreign key referring to table-predicate.id
tp-pt.predicate-table	foreign key referring to predicate-table.id

Then, the query:

  SELECT * FROM sibling('simon',NULL);

will give the following reply:

name1	name2
simon	ann

where name1 and name2 are column names defined by External Matching, and more precisely by tp-argument relation. The reply states that ann is simon's sibling.