pdf417 vb.net Part 2: Building a Microsoft Access Desktop Application in Visual Studio .NET

Generating Code128 in Visual Studio .NET Part 2: Building a Microsoft Access Desktop Application

Part 2: Building a Microsoft Access Desktop Application
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Figure 8-51. The first part of your union query in SQL view.
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Select all the text you see in this window and copy it to the Clipboard. Switch to the third empty query, and replace SELECT; with the text you copied. Remove the ending semicolon, place the insertion point at the end of the text, press the Enter key, type the word UNION, and press the Enter key again. Go to your second query, switch to SQL view, highlight all the text, and copy it to the Clipboard. Go back to the third query and paste this text at the end. Your new union query should look like Figure 8-52.
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Figure 8-52. Assembling a union query using the SQL from two other queries.
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Switch to Datasheet view to see the final result as shown in Figure 8-53. 322
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Part 1: Part Title
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Building Complex Queries
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Figure 8-53. The Datasheet view of your new union query.
Limitations on Using Select Queries to Update Data
The recordset that Access creates when you run a query looks and acts pretty much like a real table containing data. In fact, in most cases you can insert rows, delete rows, and update the information in a recordset, and Access will make the necessary changes to the underlying table or tables for you. In some cases, however, Access won t be able to figure out what needs to be changed. Consider, for example, a calculated field named Total that is the result of multiplying two fields named Quantity and Price. If you try to increase the amount in a Total field, Access can t know whether you mean to update the Quantity field or the Price field. On the other hand, you can change either the Price field or the Quantity field and then immediately see the change reflected in the calculated Total field. In addition, Access won t accept any change that might potentially affect many rows in the underlying table. For that reason, you can t change any of the data in a totals query or in a crosstab query. A Group By field is the result of gathering together one or more rows with the same value. Likewise, a Sum is most likely the result of adding values from potentially many rows. A Min or Max value might occur in more than one row. When working with a recordset that is the result of a join, Access lets you update all fields from the many side of a join but only the non-key fields on the one side, unless you have specified Cascade Update Related Fields in the relationship. Also, you cannot set or change any field that has the AutoNumber data type. For example, you can t change the ContactID field values in the tblContacts table in the LawTrack Contacts sample application. The ability to update fields on the one side of a query can produce unwanted results if you aren t careful. For example, you could intend to assign a contact to a different company. If 323
Part 2: Building a Microsoft Access Desktop Application
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You should save this query, but you can close and not save the first two queries that you used to build this. You can find this query saved as qxmplAddressesUnion in the sample database. If you want to learn more about SQL, see Article 1, Understanding SQL, on the companion CD.
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Part 1: Part Title
Microsoft Office Access 2003 Inside Out you change the company name, you ll change that name for all contacts related to the current CompanyID. What you should do instead is change the CompanyID in the tblCompanyContacts table, not the company name in the tblCompanies table. You ll learn techniques later in 13, Advanced Form Design, to prevent inadvertent updating of fields in queries. When you set Unique Values to Yes in the Query Properties window, Access eliminates duplicate rows. The values returned might occur in multiple rows, so Access won t know which one you mean to update. And finally, when Access combines rows from different tables in a union query, the individual rows lose their underlying table identity. Access cannot know which table you mean to update when you try to change a row in a union query, so it disallows all updates.
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