Dbms1
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The document discusses the commercial query language Query-by-Example (QBE) which was developed by IBM in the 1970s as a graphical query language based on relational algebra. QBE allows users to express queries by filling in example tables representing the relations and attributes being queried. The document provides examples of different types of queries that can be expressed in QBE including selection, projection, join, grouping, and aggregate queries.
Dbms1
- 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
- 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.
Editor's Notes
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- #19: Print the sname field of Sailors tuples such that there is no tuple in Reserves with the same value in the sid field.”
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- #22: 15 How can we modify this query to print the names of sailors who’ve reserved all boats?
- #23: 16 How to insert multiple tuples
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- #25: 18 This update seeks to change Joe’s age based on John’s age. Since sname is not a key, the meaning of such a query is ambiguous—should we update every John’s age, and if so, based on which Joe’s age?
- #26: 19 This query is illegal because the variable B does not appear in any positive row. Going beyond this technical objection, this QBE query is ambiguous with respect to the ordering of the two uses of ￢ . It could denote either the calculus query that we want to express or the following calculus query, which is not what we want: {I,N, T,A | I,N, T,A ∈ Sailors∧ ￢∃ Ir,Br,D ∈ Reserves ( ￢∃ B, BN,C ∈ Boats(I = Ir ∧ Br = B))} There is no mechanism in QBE to control the order in which the ￢ operations in a query are applied. (Incidentally, the above query finds all Sailors who have made reservations only for boats that exist in the Boats relation.)
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- #30: 22 It is quite expressive (relationally complete, if the update features are taken into account).