Data Base Management System Lab Guide

1. CS 222
Database Management System
Spring 2010-11
Lecture 3
Korra Sathya Babu
Department of Computer Science
NIT Rourkela
02/11/13 Lecture 3 1

2. Commercial Query Languages
• RA and RC are formal and abstract Languages developed for the
Relational model in the database technology
• Many languages were commercialized taking the foundations from the
abstract query languages
– QBE
– QUEL
– SQL
02/11/13 Lecture 3 2

3. Query-by-Example (QBE)

4. Query-by-Example (QBE)
• Developed by Moshe M. Zoolf at IBM Research
during mid 1970s (in parallel to the development of SQL)
• A graphical query language which is based on the
notion of DRC
– QBE queries can be translated to DRC directly but queries containing
aggregate operators, sorting etc cannot be converted
• Two dimensional syntax – system creates
templates of relations that are requested by users
• Queries are expressed “by example”

5. Query-by-Example (QBE)
• A “GUI” for expressing queries.
– Actually invented before GUIs.
– Very convenient for simple queries.
– Awkward for complex queries.
• QBE an IBM trademark.
– But has influenced many projects
– Especially PC Databases: Paradox,
Access, etc.

6. Syntax
• QBE Queries are posed by placing constants and variables
(example elements) into individual columns and thereby
creating an example tuple of the query result
• Variables used are domain variables
• Variable symbols are prefixed by underscore
• Commands are followed by a dot
• The field that should appear in the answer are specified by
the command P. (stands for Print (also called retrieval
command))
• Equal operator is by default . Other comparison operators
can also be used
• Link establishment between relations is done by example
elements

7. Syntax
• Aggregate functions and Groupings used by
adding prefixes are
– CNT.ALL.
– SUM.ALL.
– CNT.UNQ.ALL.
– MAX.ALL.
– MIN.ALL
– AVG.ALL
– MOD.ALL
– G. (Groupings)
• Sorting commands used are
– .DO (Descending Order)
– .AO ( Ascending Order)
• General Commands used are
– P. (Print), I. (Insert), U. (Update), D. (Delete)

8. Example
Sno Sname Ratin Age Sid bid day bid bname color
g 22 101 10/10/08 101 Interlake Blue
22 Dustine 7 45.0
22 102 10/10/08 102 Interlake Red
29 Brutus 1 33.0
22 103 10/08/08 103 Clipper Green
31 Lubber 8 55.5
22 104 10/07/08 104 Marine red
32 Andy 8 25.5
31 102 10/10/08 Instance of Boats
58 Rusty 10 35.0
31 103 11/06/08
64 Horatio 7 35.0
31 104 11/12/08
71 Zorba 10 16.0
64 101 09/05/08
74 Horatio 9 35.0
85 Art 3 25.5 64 102 09/08/08
95 Bob 3 63.5 74 103 09/08/08
Instance of Instance of Reserves
Sailors
02/11/13 Lecture 3 8

9. `Example Tables’ in QBE
Relation name attributes
• QBE has its own screen Reserves sid bid day
editor
• Users specify a query by
filling in example tables, or
skeletons; we will use Boats bid bname color
these skeletons in our
examples.
Sailors sid sname rating age

10. `Creating Tables’ in QBE
Dictionary entry for Relation Reserves
I.P.I. Reserves sid bid day
Represent the dictionary entry of file columns of table Reserves

11. Basics
• To print names and ages of all sailors:
Sailors sid sname rating age
P._N P._A
Print all fields for sailors with rating > 8, in
ascending order by (rating, age):
Sailors sid sname rating age
P. AO(1). >8 AO(2).
QBE puts unique new variables in blank
columns. Above query in DRC (no ordering):
{ I, N , T , A | I, N , T , A ∈ Sailors ∧ T > 8}

12. And/Or Queries
• Names of sailors younger than 30 or older
than 20: Sailors sid sname rating age
P. < 30
P. > 20
Names of sailors younger than 30 and older than
20: Sailors sid sname rating age
_Id P. < 30
_Id P. > 20
Names of sailors younger than 30 and rating > 4:
Sailors sid sname rating age
_Id P. > 4 < 30

13. Duplicates
• Single row with P: Duplicates not eliminated
by default; can force elimination by using
UNQ.
Sailors sid sname rating age
UNQ. P. < 30
Multiple rows with P: Duplicates eliminated by
default! Can avoid elimination by using ALL.
Sailors sid sname rating age
ALL. _Id P. < 30
_Id P. > 20

14. Join Queries
• Names of sailors who’ve reserved a boat for
11/06/2008 and are older than 25 (note that
dates and strings with blanks/special chars
are quoted):
Sailors sid sname rating age
_Id P._S > 25
Reserves sid bid day
_Id ‘8/24/96’
Joins accomplished by repeating variables.

15. Join Queries (Contd.)
• Colors of boats reserved by sailors who’ve
reserved a boat for 8/24/96 and are older
than 25 :
Sailors sid sname rating age
_Id _S > 25
Reserves sid bid day
_Id _B ‘8/24/96’
Boats bid bname color
_B ‘Interlake’ P.

16. Join Queries (Contd.)
• Names and ages of sailors who’ve reserved
some boat that is also reserved by the sailor
with sid = 22:
Sailors sid sname rating age
_Id P. P.
Reserves sid bid day
22 _B
_Id _B

17. Unnamed Columns
• Useful if we want to print the result of an
expression, or print fields from 2 or more
relations.
– QBE allows P. to appear in at most one table!
Sailors sid sname rating age
_Id P. _R _A P._D P.(_R/_A)
Reserves sid bid day
_Id _D

18. “Negative Tables”
• Can place a negation marker in the
relation column:
Sailors sid sname rating age
_Id P._S
Reserves sid bid day
¬ _Id _B
Variables appearing in a negated table must
also appear in a positive table!

19. Aggregates
• QBE supports AVG, COUNT, MIN, MAX, SUM
– None of these eliminate duplicates, except COUNT
– Also have AVG.UNQ. etc. to force duplicate
elimination
Sailors sid sname rating age
_Id G. G.P.AO _A P.AVG._A
The columns with G. are the group-by fields; all
tuples in a group have the same values in these
fields.
— The(optional) use of .AO orders the answers.
— Every column with P. must include G. or an
aggregate operator to get the aggregates.

20. Conditions Box
• Used to express conditions involving 2 or
more columns, e.g., _R/_A > 0.2.
• Can express a condition that involves a
group, similar to the HAVING clause in SQL:
Sailors sid sname rating age CONDITIONS
G.P. _A AVG._A > 30
Express conditions involving AND and OR:
Sailors sid sname rating age CONDITIONS
P. _A 20 < _A AND _A < 30

21. Find sailors who’ve reserved all boats
• A division query; need aggregates (or update
operations) to do this in QBE.
Sailors sid sname rating age
P.G._Id
Reserves sid bid day CONDITIONS
_Id _B1 COUNT._B1= COUNT._B2
Boats bid bname color
_B2

22. Inserting Tuples
• Tuple insertion:
Sailors sid sname rating age
I. 74 Janice 7 14

23. Delete and Update
• Delete all reservations for sailors with rating < 4
Sailors sid sname rating age
_Id <4
Reserves sid bid day
D. _Id
Increment the age of the sailor with sid = 74
Sailors sid sname rating age
74 U._A+1

24. Restrictions on Update Commands
• Cannot mix I., D. and U. in a single example
table, or combine them with P. or G.
• Cannot insert, update or modify tuples using
values from fields of other tuples in the same
table. Example of an update that violates this
rule:
Sailors sid sname rating age
john _A
joe U._A+1
Should we update every Joe’s age?
Which John’s age should we use?

25. Find sailors who’ve reserved all boats
(Again!)
• We want to find sailors _Id such that there is
no boat _B that is not reserved by _Id:
Sailors sid sname rating age
_Id P._S
Boats bid bname color
¬ ¬
Reserves sid bid day
_B _Id _B
Illegal query! Variable _B does not appear in a
positive row. In what order should the two
negative rows be considered? (Meaning
changes!)

26. A Solution Using Views
• Find sailors who’ve not reserved some boat _B:
Sailors sid sname rating age BadSids sid
_Id P._S I. _Id
Boats bid bname color
¬
Reserves sid bid day
_B _Id _B
Next, find sailors not in this `bad’ set:
¬
Sailors sid sname rating age BadSids sid
_Id P._S _Id

27. Advantages of QBE
• User-friendly
• Order of the row is immaterial
• Highly non procedural query language
• Several ways to represent a query
• QBE is interpreter based so debugging is easy

28. Disadvantages of QBE
• QBE doesn’t support complete views
• QBE doesn’t have a preprocessor for
optimization
• Implementation of QBE is not complete but
theoretically its complete

29. Summary
• QBE is an elegant, user-friendly query language
based on DRC.
• Simple queries are especially easy to write in
QBE, and there is a minimum of syntax to learn.
• Has influenced the graphical query facilities
offered in many products, including Borland’s
Paradox and Microsoft’s Access.