the university of akron dept of business technology computer information systems

The University of AkronThe University of AkronDept of Business TechnologyDept of Business Technology

Computer Information SystemsComputer Information Systems

The Relational Model:The Relational Model:

Query-By-Example (QBE)Query-By-Example (QBE)

2440: 1802440: 180Database ConceptsDatabase Concepts

Instructor: Enoch E. DamsonInstructor: Enoch E. Damson

2The Relational Model: QBE

Query-by-Example (QBE)Query-by-Example (QBE) Query Query

Questions represented in a way the DBMS Questions represented in a way the DBMS can recognize and processcan recognize and process

QBE QBE

Visual approach to writing queriesVisual approach to writing queries

Used in MS-AccessUsed in MS-Access


Figure 2.3: An Example of Simple Queries


Figure 2.4: Simple Queries…


Simple CriteriaSimple Criteria Criteria – conditions that data must satisfyCriteria – conditions that data must satisfy

Criterion – a single conditionCriterion – a single condition

To display specific query results, enter the To display specific query results, enter the condition in the appropriate column in the condition in the appropriate column in the design griddesign grid


Query with Simple Criteria


Comparison (Relational) Comparison (Relational) OperatorsOperators

Finds something other than an exact matchFinds something other than an exact match

Comparison operators are:Comparison operators are:

= (equal to)= (equal to)

> (greater than)> (greater than)

< (less than)< (less than)

>= (greater than or equal to)>= (greater than or equal to)

<= (less than or equal to)<= (less than or equal to)

NOT (not equal to)NOT (not equal to)


Compound CriteriaCompound Criteria

Combines comparison operatorsCombines comparison operators

Many languages use AND or OR between Many languages use AND or OR between the separate criteriathe separate criteria

In an AND criterion, both criteria must be In an AND criterion, both criteria must be truetrue

In an OR criterion, the overall criterion is In an OR criterion, the overall criterion is true if either of the individual criteria is true if either of the individual criteria is truetrue


Query Using AND Criteria


Query Using OR Criteria


Computed FieldsComputed Fields You can include calculated fields that are You can include calculated fields that are

not in the database in queriesnot in the database in queries

Computed field (calculated field) – a field Computed field (calculated field) – a field that is the result of a calculation using one that is the result of a calculation using one or more existing fieldsor more existing fields

In a query that uses computed fields, if a In a query that uses computed fields, if a field name contains spaces you must field name contains spaces you must enclose it in square bracketsenclose it in square brackets


Query Using Computed Field


Calculating StatisticsCalculating Statistics

CountCount

SumSum

Avg (average)Avg (average)

Max (largest value)Max (largest value)

Min (smallest value)Min (smallest value)

StDev (standard StDev (standard deviation)deviation)

Var (variance)Var (variance)

FirstFirst

LastLast

Built-in statistics (called aggregate functions in Access) include:


Query to Calculate an Average


GroupingGrouping

Grouping – creating groups of records Grouping – creating groups of records that share some common characteristicthat share some common characteristic

Functions can be used in combination Functions can be used in combination with grouping where statistics are with grouping where statistics are calculated for groups of recordscalculated for groups of records


SortingSorting Sorting – listing records in a query’s results in Sorting – listing records in a query’s results in

a particular waya particular way It is possible to sort using more than one fieldIt is possible to sort using more than one field Sort Key – the field on which records are Sort Key – the field on which records are

sortedsorted Major sort key (primary sort key) – the more Major sort key (primary sort key) – the more

important fieldimportant field Minor sort key (secondary sort key) – the less Minor sort key (secondary sort key) – the less

important fieldimportant field Major sort key is on the left of the grid and the Major sort key is on the left of the grid and the

minor sort key is on the rightminor sort key is on the right


Query to Sort Records


Query to Sort on Multiple Keys


Query to Sort on Multiple Keys…


Joining TablesJoining Tables Queries to select data from more than one tableQueries to select data from more than one table

Join the tables based on matching fields in Join the tables based on matching fields in corresponding columnscorresponding columns

In an Access query, a join line between In an Access query, a join line between matching fields in the two tables will be created matching fields in the two tables will be created indicating how the tables are relatedindicating how the tables are related

When joining multiple tablesWhen joining multiple tables

Add all the tables involved to the upper paneAdd all the tables involved to the upper pane

Add the query results grid in the desired orderAdd the query results grid in the desired order


Query to Join Table


Query to Join Tables…


Update QueryUpdate Query

Update query – a query that changes dataUpdate query – a query that changes data

Makes a specified change to all records Makes a specified change to all records satisfying the criteria in the querysatisfying the criteria in the query

In Access, a new row is created that is In Access, a new row is created that is used to indicate how to update the data used to indicate how to update the data selected by the queryselected by the query


Update Query


Delete QueryDelete Query Queries can be used to delete one or more Queries can be used to delete one or more

records at a timerecords at a time

Delete query – deletes all the records Delete query – deletes all the records satisfying the criteria entered into the satisfying the criteria entered into the queryquery

When you change the Query type to Delete When you change the Query type to Delete Query, an extra row, called a Delete row, Query, an extra row, called a Delete row, is added to the design gridis added to the design grid


Delete Query


Make-Table QueryMake-Table Query

Queries can be used to create a new table Queries can be used to create a new table in either the current database or in a in either the current database or in a separate databaseseparate database

Make-table query – creates a new table Make-table query – creates a new table using the query resultsusing the query results

The data added to the new table is The data added to the new table is separate from the original table in which separate from the original table in which it appearsit appears


Make-Table Query


Make-Table Query…

the university of akron dept of business technology computer information systems

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