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Chapter 7Query-By-Example

byMonica Chan

CS157BProfessor Lee

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Structure of the chapter

7.1 Introduction to Microsoft Access Queries

7.2 Building Select Queries Using QBE7.3 Using Advanced Queries7.4 Changing the Content of Tables

Using Action Queries

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Objectives IIn this chapter you will learn:

The main features of Query-By-Example (QBE). The types of query provided by the Microsoft Access DBMS QBE facility. How to use QBE to build queries to select fields and records.

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Objectives IIHow to use QBE to target single or multiple tables. How to perform calculations using QBE. How to use advanced QBE facilities including parameter, find matched, find unmatched, crosstab, and autolookup queries. How to use QBE action queries to change the content of tables.

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QBE facilities

QBE was developed originally by IBM in 1970s to help users in their retrieval of data from a databaseQBE is now provided by most popular Data Base Management Systems including Microsoft Access

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Microsoft Access 2000Visual approach for retrieving information from database using query templatesWhen a query is created using QBE, Microsoft constructs the equivalent SQL statement in the background

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Microsoft Access 2000

Many of the Microsoft Access SQL statements displayed in this chapter do not comply with the SQL standard presented in Chapters 5 and 6

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7.1 Introduction to Microsoft Access QueriesWhen we create or open a database using Microsoft Access, the Database window is displayed showing the objects such as tables, forms, queries, and reports in the database.

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DreamHome database

See Appendix A for users’ requirements specification.

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Introduction to Microsoft Access

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Summary of Microsoft Access Query Types

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Create a new queryWhen we create a new query, Microsoft Access displays the New Query dialog boxThe dialog box shows the options

Design View for building a new query from scratchWizards for help to build the queries such as select, crosstab, duplicates, or unmatched queries

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Figure 7.2 Microsoft Access New Query dialog box

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7.2 Building Select Qqueries Using QBESelect query- is the most common type of query- retrieves the data from one or more

tables and displays the results in a datasheet (similar to a spreadsheet) where we can update and edit the records with some restrictions

- can group records and calculate sums, counts, averages, and other types of total

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Select Query windowSelect Query window is a graphical QBE toolBecause of its graphical features, we can use a mouse to select, drag, or manipulate objects in the window to define an example of the records we want to seeWe specify the fields and records we want to include in the query in the QBE grid

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Figure 7.3 (a) QBE grid to retrieve the propertyNo, city, type, and rent fields of the PropertyForRent table; (b) resulting datasheet; (c) equivalent SQL statement.

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AND operator

If the expressions are in different cells in the same row, which means only the records that meet the criteria in all the cells will be returned.

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Figure 7.4 (a) QBE grid of select query to retrieve the properties in Glasgow with a rent between £350 and £450; (b) resulting datasheet; (c) equivalent SQL statement.

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OR operator

If the expressions are in different rows of the design grid, which means records that meet criteria in any of the cells will be returned.

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Figure 7.5 (a) QBE grid of select query to retrieve the properties in Glasgow with a rent between £350 and £450 and all properties in Aberdeen; (b) resulting datasheet; (c) equivalent SQL statement.

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Creating Multi-Table Queries

In a database that is correctly normalized, related data may be stored in several tables.

Microsoft Access automatically shows a join line (a join line is only shown with a symbol if relationship has been previously established between two tables) between tables in the QBE grid if they contain a common field.

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Figure 7.6 (a) QBE grid of multi-table query to retrieve the first and last names of owners and the property number and city of their properties; (b) resulting datasheet; (c) equivalent SQL statement.

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Totals (Aggregate) queriesMS Access provides aggregate functions such as Sum, Avg, Min, Max, and CountTo access these functions, we change the query type to Totals, which results in the display of an additional row called Total in the QBE gridWhen a totals query is run, the resulting datasheet is a snapshot, a set of records that is not updatable

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Figure 7.7 QBE grid of totals query to calculate the number of properties for rent in each city; (b) resulting datasheet; (c) equivalent SQL statement.

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Figure 7.8 (a) QBE grid of select query to calculate the yearly rent for each property; (b) resulting datasheet; (c) equivalent SQL statement.

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7.3 Using Advanced Queries

MS Access provides a range of advanced queries such as

- parameter queries- crosstab queries- find duplicate queries- find unmatched queries

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Parameter queryA parameter query displays one or more predefined dialog boxes that prompt the user for the parameter value(s) or criteria.Parameter queries are created by entering a prompt enclosed in square brackets in the Criteria cell for each field we want to use as a parameter.

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Figure 7.9 (a) QBE grid of example parameter query; (b) dialog boxes for first and last name of owner; (c) resulting datasheet; (d) equivalent SQL statement.

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Crosstab queryA crosstab query can be used to summarize data in a compact spreadsheet format which enables users of large amounts of summary data to more easily identify trends and to make comparisons.The result datasheet is a snapshot.

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Figure 7.10 (a) QBE grid of example totals query; (b) resulting datasheet; (c) equivalent SQL statement.

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Figure 7.11 (a) QBE grid of example crosstab query; (b) resulting datasheet; (c) equivalent SQL statement.

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Find Duplicates queryThe Find Duplicates Query Wizard is used to determine if there are duplicate records in a table or determine which records in a table share the same value.

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Figure 7.12 (a) QBE for example Find Duplicates query; (b) resulting datasheet; (c) equivalent SQL statement.

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Find Unmatched queryThe Find Unmatched Query Wizard is used to find records in one table that do not have related records in another table

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Figure 7.13 (a) QBE grid of example Find Unmatched query; (b) resulting datasheet; (c) equivalent SQL statement.

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Autolookup queryIt can be used to automatically fill in certain field values for a new record.When we enter a value in the join field in the query or in a form based on the query, MS Access looks up and fills in existing data related to that value, and displays an error if no matching data is found.

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Figure 7.14 (a) QBE grid of example autolookup query (b) datasheet based on autollokup query; (c) equivalent SQL statement.

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7.4 Changing the Content of Tables Using Action

QueriesWhen we run a select query, MS Access

displays the resulting datasheet. As the datasheet is updatable, we can make changes to the data; however, we must make the changes record by record.

Action query can be used to make changes to many records at the same time.

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Four types of action queries

Make-Table Action QueryDelete Action QueryUpdate Action QueryDelete Action Query

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Make-Table Action QueryIt creates a new table from all or part of the data in one or more tables.The newly created table can be saved to the currently opened database or exported to another database.The data in the new table does not inherit the field properties including the primary key from the original table.

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Figure 7.15 (a) Make-Table dialog box; (b) QBE grid of example make-table query; (c) warning message; (d) resulting datasheet; (e) equivalent SQL statement.

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Delete Action QueryIt deletes a group of records from one or more tables.We can use a single delete query to delete records form a single table, from multiple tables in a 1:1 relationship, or from multiple tables in a 1:M relationship with referential integrity set to allow cascading deletes.

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Figure 7.16 (a) QBE grid of example delete action query; (b) warning message; (c) resulting PropertyForRent and Viewing datasheets with records deleted; (d) equivalent SQL statement.

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Update Action QueryIt makes global changes to a group of records in one or more tables.

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Figure 7.17 (a) QBE grid of example update action query; (b) warning message; (c) resulting datasheet; (d) equivalent SQL statement.

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Append Action QueryIt is used to insert records from one or more source tables into single target table in the same database or in another database.

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Figure 7.18 (a) Append dialog box; (b) QBE grid of example append action query; (c) warning message;

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Figure 7.18 (d) the NewOwner table and the PrivateOwner table with the newly appended records; (e) equivalent SQL statement.

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ReferenceBegg C. (2002). Database Systems.

Pearson Education Limited