Query-by-Example (QBE) Presented by: Wong, Lok Man CS 157A Dr. Sin Min Lee.
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Transcript of Query-by-Example (QBE) Presented by: Wong, Lok Man CS 157A Dr. Sin Min Lee.
Query-by-Example (QBE)Query-by-Example (QBE)
Presented by: Wong, Lok Man
CS 157A
Dr. Sin Min Lee
History of QBEHistory of QBE
History:• QBE is the name of both a data-manipulation
language and an early database system that included this language
• QBE was developed at IBM’s T.J. Watson Research Center (1970’s)
ContentContent
• QBE Basic Structure• Queries on One Relation• Queries on Several Relations• Condition Box• Result Relation• Ordering of the Display of Tuples• Aggregate Operations• Modification of Database• QBE in Microsoft Access
Basic StructureBasic Structure• QBE has two dimensional syntax:
1. Queries look like tables
2. QBE queries are expressed “by example”• Non-procedural• Queries are expressed using skeleton tables
branch Branch-name Branch-city assets
Basic StructureBasic Structure
• User selects skeletons needed• User fills in the skeletons with example row• Example row consists of constants & example elements,
which are domain variables• Constants appear without any qualification• QBE uses an underscore character “_” before domain
variables, as in “_x”
Queries on One RelationQueries on One Relation
Ex: Find all loan numbers at the Perryridge branchBring up the skeleton of the loan relation
P. – before the variable to display the value
X – a variable
Perryridge – branch-name
Loan loan-number branch-name amount
P._x Perryridge
Queries on One Relation (cont.)Queries on One Relation (cont.)
Ex: Suppress duplicate elimination - insert “ALL”
Ex: display entire loan relation – place single P.
loan loan-number branch-name amount
P.ALL Perryridge
loan loan-number branch-name amount
P.
Queries on One Relation (cont.)Queries on One Relation (cont.)
Ex: Find loan numbers of all loans with amount more than $700. Use arithmetic operations (=, <, >, ¬) With no variable, displays all attributes
Ex: Find names of all branches not in Brooklyn
loan loan-number branch-name amount
P. >700
branch branch-name branch-city assets
P. ¬Brooklyn
Queries on One Relation (cont.)Queries on One Relation (cont.)
Ex: Find the loan numbers of all loans made jointly to Smith and Jones. Only one P. is needed.
Ex: Find all customers who live in the same city as Jones
customer customer-name customer-street customer-city
P._x _y
Jones _y
borrower Customer-name loan-number
“Smith” P._x
“Jones” _x
Queries on Several Relations Queries on Several Relations
• QBE allows queries to span several different relations
• The connection among the various relations are achieved through variables that force certain tuples to have the same value on certain attributes
Queries on Several Relation (cont.)Queries on Several Relation (cont.)
Ex: Find the names of all the customers that have a loan from the Perryridge branch
loan loan-number branch-name amount
_X Perryridge
borrower customer-name loan-number
P._y _X
Queries on Several Relation (Cont.)Queries on Several Relation (Cont.)
Ex: Find the names of all customer with both and an account and a loan at the bank
The “¬” symbol represents “there does not exist”.A “¬” under an attribute name stands for “not
equal”.
depositor customer-name account-number
P._x
borrower customer-name loan-number
¬ _x
Queries on Several Relation (Cont.)Queries on Several Relation (Cont.)
Ex: Display all customer-name values that appear in at least two tuples, with the second tuple having an account-number different from the first.
depositor customer-name account-number
P._x _y
_x ¬_y
Condition BoxCondition Box• QBE includes a condition box feature that allow the
expression of general constraints over
any of the domain variables.• Logical operators in words and and or, or as symbols “&”
and “|”, appear in a condition box
Ex: Find the loan numbers of all loans made to Smith, to Jones, or both
borrower customer-name Loan-number
_n P._x
condition
_n = Smith or _n = Jones
Condition Box (Cont.)Condition Box (Cont.)
Ex: Find all branches that have assets greater than those of at least one branch located in Brooklyn
branch branch-name branch-city assets
P._x _y
Brooklyn _z
conditions
_y > _z
Condition Box (Cont.)Condition Box (Cont.)
Ex: Find all account numbers of account with a balance between $1000 and $1500, but not exactly $1255
account account-number branch-name balance
P._y _x
conditions
_x = ( > 999 and < 1501 and ¬ 1255)
Result RelationResult Relation
• If the result of a query includes attributes from several relation schemas, we can declare a temporary result relation that includes all the attributes of the result.
Ex: Create a skeleton table, called result, with attributes customer-name, account-number, and balance, at the Perryridge branch. (Two Steps)
1. account account-number branch-name balance
_y Perryridge _z
depositor customer-name account-number
_x _y
Result Relation (Cont.)Result Relation (Cont.)
Once the new skeleton table is created, write the query.
2.
result customer-name
account-number
balance
result customer-name
account-number
balance
P. _x _y _z
Ordering of the Display TuplesOrdering of the Display Tuples
• To gain control over the order in which tuples in a relation are displayed, indicate AO (ascending order) or Do (descending order).
• To specify the order in which the sorting should be carried, add an integer surrounded by parentheses after sort operator (AO(1) or DO(2)).
Ordering of the Display Tuples (Cont.)Ordering of the Display Tuples (Cont.)
Ex: List all account numbers at the Downtown branch in ascending alphabetic order with their respective account balances in descending order.
account account-number branch-name balance
P.AO(1) Downtown P.DO(2)
Aggregate OperationsAggregate Operations
• Aggregate Operators include AVG, MIN, MAX, SUM, and CNT
• We must postfix these operators with “ALL.” to create a multiset on which the aggregate operation is evaluated
• Operator “UNQ.” is to specify that duplicates are to be eliminated
• To compute functions on groups of tuples, then use “G.”.
Aggregate Operations (Cont.)Aggregate Operations (Cont.)
Ex: Find the total balance of all the accounts maintained at the Downtown branch
Ex: Find the total number of customers who have an account at the bank
account account-number branch-name balance
Downtown P.SUM.ALL.
depositor customer-name account-number
P.CNT.UNQ.
Aggregate Operations (Cont.)Aggregate Operations (Cont.)
Ex: Find the average balance at each branch
account account-number branch-name balance
P.G. P.AVG.ALL._x
Ex: Find all customers who have accounts at each of the branches located in Brooklyn
depositor customer-name account-number
P.G._X _y
account account-number branch-name balance
_y _z
branch branch-name branch-city assets
_z Brooklyn
_w Brooklyn
conditions
CNT.UNQ._z = CNT.UNQ._w
Modification of the DatabaseModification of the Database
• Deletion, uses “D.” replacing “P.”
• Insertion, uses “I.” replacing “P.”
• Updates, uses “U.” replacing “P.”
DeletionDeletion
Ex: Delete customer Roy
Ex: Delete the branch-city value of the branch whose name is “San Jose”
depositor customer-name account-number
D. Roy
branch branch-name branch-city assets
San Jose D.
Deletion (Cont.)Deletion (Cont.)
Ex: Delete all accounts at all branches located in Brooklyn
depositor customer-name account-number
D. _y
branch branch-name branch-city assets
_x Brooklyn
account account-number branch-name balance
D. _y _x
InsertionInsertion
Ex: Insert the fact that account AM-8976 at the Downtown branch has a balance of $900.
Ex: Insert information into the branch relation about a new branch with name “Capital”, city “Queens,” and asset “200,000”
account account-number branch-name balance
I. AM-8976 Downtown 900
branch branch-name branch-city assets
I. Capital Queens 200,000
Insertion (Cont.)Insertion (Cont.)
Ex: Insert as a gift, for all loan customers of the Downtown branch, a new $300 savings account for every loan account that they have, with the loan number serving as the account number for the savings account.
Insertion (Cont.)Insertion (Cont.)account account-number branch-name balance
I. _x Downtown 300
depositor customer-name account-number
I. _y _x
loan loan-number branch-name amount
_x Downtown
borrower customer-name Loan-number
_y _x
UpdatesUpdatesEx: Update the asset value of the San Jose branch
to $100,000,000.
Ex: Update all balances with a 5% increase.
Use “_x” to hold the old balances
branch branch-name branch-city assets
San Jose U.100,000,000
account account-number branch-name balance
U._x * 1.05
QBE in Microsoft AccessQBE in Microsoft Access
• Original QBE was designed for a text-based display environment
• Access QBE is a graphical display environment, called graphical query-by-example (GQBE)
• GQBE version attributes are written one below the other, instead of horizontally
• Instead of shared variable in QBE, GQBE uses lines to link attributes of two tables
QBE in Microsoft Access (Cont.)QBE in Microsoft Access (Cont.)
• In GQBE links are automatically created, on the basis of the attribute name
• In GQBE attributes printed in a separate box is specified, called the design grid, instead of using “P.” for QBE.
THE END!!!THE END!!!
THANKS FOR STAYING AWAKE!
QUIZ TIME!!!