Querying a Database

Microsoft Office Access 2003

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Query window in Design view

• The Query window in Design view allows you to specify the results you want for a query.

• In the query Design view, you can specify which fields you want to be included in your query results.

• You can also control what records are displayed in the query by specifying select query criteria.

• Each column in the design grid represents a field that will be used in the query.

• You can run the query at anytime to view the results according to the current specifications.

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Add fields to a query in Design view

• The design grid is used to specify the fields and records you want to see.

• Add fields to the design grid by double-clicking the field in the field list or by dragging the field to the design grid. – Remember that the results of a query provide a

temporary view of the data– The contents displayed in a query are a result of

the fields you select and the criteria you specify in the query design grid

• When you use the query design window, you use Query By Example (QBE). When you use QBE, you give Access an example of the information you wish to see when you run your query.

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The Query Design view window

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Create, run, and save queries

• From the fields list, either drag or double-click fields that you want included in the query. – A query result differs from a table's datasheet view in

that only selected fields are displayed • If you move a field to the design grid and then

you want to remove it, you can click on the field's column and then press the delete key.

• The results of the query will be displayed in order by the primary key of the table unless you specify another sort order.

• You can save the query and give it a name by pressing the save button on the toolbar.

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Selecting, displaying and sorting fields

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A sample query datasheet

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Update data using a query

• You can use the query datasheet to update data in a table.

• It is important to realize that the query datasheet is a temporary view of the data. However, when you update data by means of the query datasheet, the updates are placed directly into the underlying table.

• You can only update fields that are represented in the query.

• To observe the actual changes made to the table, close the query and open the table in datasheet view.

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Access is a relational database

• A database is almost always a collection of tables. Access is a relational database management system that allows you to form relationships between the tables.

• When you form a relationship between tables, you are joining the tables.

• Tables are joined on common fields between the tables.

• When tables are joined, you can view data from both tables as if the tables were one combined table.

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A one-to-many relationship

• Tables can be joined in three ways; one-to-one, one-to-many, and many-to-many.

• A one-to-many relationship exists when one table has many records associated with a given value but the related table has only one record for that value.– The table with one record is called the primary table– The table with many records is called the related table

• Queries can be defined to use the relationship to extract data from both tables in a single query.

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Using referential integrity

• When dealing with related tables, you need to decide if you want to enforce referential integrity.

• Referential integrity allows you to maintain the integrity between related tables.

• The rules associated with referential integrity specify that when you add a record to a related table, there must be a matching record in the primary table.

• If you choose to enforce referential integrity, you can insure that you will not have orphaned records (records that have no matching record in the primary table).

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Use cascaded updates and cascaded deletes

• In addition to referential integrity, you can also tell Access to implement cascaded updates and cascaded deletes.

• If you choose cascaded updates, making a change in a field that is common to two related tables will cause the update to be made in both tables.

• Cascade deletes is similar. If you delete a field that is common to two tables, the deletion will take place in both tables.

• You should carefully consider whether you want to implement these features, as they can have dramatic effects on your data.

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Selecting the tables for a relationship

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Setting relationship options

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The Relationships window

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Sort data in a query

• You can control the order of data displayed in a query by altering the queries sort key.

• Choose a field on which you want the data to be sorted and then specify whether you want the data sorted in ascending or descending order.

• Specify multiple fields as the sort key if you want to arrange data in sorted order within groups of data.

• Specify sort criteria for the query in the query design grid.

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Sort results by data type

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Sort single or multiple fields

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Filter data in a query

• Sometimes you may want to isolate a certain portion of data in your query results.

• You can set a filter, or a set of restrictions, to specify what records you want to view in the results. Only those records that match the criteria will display in the results datasheet.

• There are two methods used to make your selections: Filter By Selection or Filter By Form.

• If you no longer want to view the data with the filter applied you can remove the filter.

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Using Filter By Selection

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Specify an exact match condition in a query

• You can specify an exact match condition, which allows you to specify that only records that match your criteria will be displayed.

• To specify an exact match, use the criteria row in your query design grid.

• When you run the query after specifying an exact match, only those records that meet the criteria will be displayed.

• An exact match condition specifies that the value in the specified field must match the condition exactly.

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Exact match operators and results

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Change a datasheet's appearance

• You can make several alterations to the way a query datasheet is displayed.

• You can change the font type, the font size, and the size of the columns.

• To alter the font of the datasheet:– Click Format on the menu bar– Select Font from the dropdown menu– In the Font dialog box, select the font you want and

also change the size of the font

• Columns can be resized individually or you can select and resize all of them at one time.

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Use a comparison operator to match a range of values

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Use the And and Or logical operators

• When you need to use multiple conditions for a query, you can use the logical operators to combine conditions.

– The And logical operator specifies that both conditions must be met

– The Or logical operator specifies that one or the other of the conditions must be met

• When you enter two conditions on the same row of the query design grid, an And condition is created.

• If you enter two conditions that are on separate rows, an Or condition is created.

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Illustration of And and Or logic

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Creating And conditionin the design grid

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Use multiple undo and redo

• In Access 2003, you can Undo and Redo multiple actions at one time.

• While working in design view for tables, queries, forms, and reports, you can use the undo recent actions button.

• The Undo button on the design toolbar has an associated list box. You can open the list box and then select the action you want to undo.

• If you undo an action and then change your mind, you can also redo actions by the same method, using the Redo button.

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Using the Undo actions button

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Perform calculations in a query using calculated fields and expressions

• Queries can be built to perform a calculation as part of the query.

• Expressions can be entered into the query design grid.

• Make certain that you are following the rules of precedence.

• To perform a calculation in a query, you must add a calculated field to the query design.

• You have three options for entering expressions: – Enter the expression directly into the field text box– Enter the expression in the Zoom box– Enter the expression in the Expression Builder

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The Expression Builder dialog box

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A calculated field in the query datasheet

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Using Aggregate Functions