1. Managing XML and Semistructured Data
Lecture 5: Query Languages -
Lorel and UnQL
Prof. Dan Suciu
Spring 2001

2. In this lecture A core query language Lorel UnQL Resources:
UnQL: A Query Language and Algebra for Semistructured Data Based on Structural Recursion Buneman, Fernandez, Suciu. VLDBJ 2000
The Lorel Query Language for Semistructured Data  by Abiteboul, Quass, McHugh, Widom, Wiener, in International Journal on Digital Libraries, 1997.

3. A Core Query Language Will illustrate with: DB = . . . 1976 Database
biblio
&o1
book
paper
book
&o12
&o24
&o29
. . .
title
author
date
author
title
author
date
&o52
&96
&25
&30
1976
Database
Systems
&o47
&o48
&o50
Roux
Combalusier
Smith
1999
Database
Systems

4. Query 1: SELECT author: X FROM biblio.book.author X . . .
answer
&o1
&o12
&o24
&o29
&96
&30
paper
book
author
date
title
biblio
&o47
&o48
&o50
&o52
&25
Smith
1999
Database
Systems
Roux
Combalusier
1976
. . .
author
Answer = {author: “Smith”,
author: “Roux”,
author: “Comalusier”}
author
author

5. Query 2: SELECT row: X FROM biblio._ X WHERE “Smith” in X.author . . .
answer
&o1
&o12
&o24
&o29
&96
&30
paper
book
author
date
title
biblio
&o47
&o48
&o50
&o52
&25
Smith
1999
Database
Systems
Roux
Combalusier
1976
. . .
Answer = {row: {author:“Smith”,
date: 1999,
title: “Database…”}, row: … }
row

6. SELECT row: ( SELECT author: Y FROM X.author Y) FROM biblio.book X
Query 3:
row
answer
&o1
&o12
&o24
&o29
&96
&30
paper
book
author
date
title
biblio
&o47
&o48
&o50
&o52
&25
Smith
1999
Database
Systems
Roux
Combalusier
1976
. . .
&a1
row
Answer = {row: {author:“Smith”}, row: {author:“Roux”, author:“Combalusier”,}, }
&a2
author
author
author

7. SELECT ( SELECT row: {author: Y, title: T} FROM X. author Y, X
SELECT ( SELECT row: {author: Y, title: T} FROM X.author Y, X.title T) FROM biblio.book X WHERE “Roux” in X.author
Query 4:
row
answer
&o1
&o12
&o24
&o29
&96
&30
paper
book
author
date
title
biblio
&o47
&o48
&o50
&o52
&25
Smith
1999
Database
Systems
Roux
Combalusier
1976
. . .
Answer = {row: {author:“Roux”,
title: “Database…”}, row: {author:“Combalusier”,
title: “Database…”}, }
&a1
row
author
&a2
title
title
author
(Query has typo in the book )

8. Formal Semantics SELECT E[X1, …, Xn] FROM F WHERE C
Given query Q = and database DB
Answer(Q,DB) is defined in two steps:
Step 1: compute all bindings:
Cij are node oids or atomic values
Must satisfy paths in F
Must satisfy conditions in C
Step 2: answer is E[C11, …, C1n]  …  E[Cm1, …, Cmn] X1 X2 … Xn
Ci1
Ci2
Cin

9. Formal Semantics
When E has nested subqueries, apply semantics recursively
Note: so far we have dealt with an unordered model
What do we need to do for order ?
Complexity: PTIME in |DB| (not in |Q|).

10. Lorel
Minor syntactic differences in regular path expressions (% instead of _, # instead of _*)
Common path convention: becomes:
SELECT biblio.book.author FROM biblio.book WHERE biblio.book.year = 1999
SELECT X.author FROM biblio.book X WHERE X.year = 1999

11. Lorel Existential variables:
What happens with books having multiple authors ? Author is existentially quantified:
SELECT biblio.book.year FROM biblio.book WHERE biblio.book.author = “Roux”
SELECT X.year FROM biblio.book X, X.author Y WHERE Y = “Roux”

12. Lorel Path variables. @P in: Constructing new results
What happens on graphs with cycles ?
Constructing new results
Several default rules
Casting between datatypes
Very useful in practice
SELECT P FROM X

13. UnQL Patterns: Equivalent to:
SELECT row: X WHERE {biblio.book: {author “Roux”, title X}} in DB,
SELECT row: X FROM biblio.book Y, Y.author Z, Y.title X WHERE Z=“Roux”

14. UnQL
Label variables:
“find all publication types and their titles where Roux is an author”
SELECT row: {type: L, title : X} WHERE {biblio.L: {author “Roux”, title X}} in DB,

15. UnQL Unrestricted use of label variables creates problems:
SELECT row: {type: L, title : Y} WHERE {biblio.(book|L).title X} in DB,
SELECT row: {type: L, title : Y} WHERE {biblio.(L)*.title X} in DB,

16. UnQL In UnQL regular path expressions cannot contain label variables:
Pat ::= Var | Const | {L1:Pat1, …, Ln:Patn} L ::= RegularPathExpression | LabelVariable