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Part 2: Building a Microsoft Access Desktop Application
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Part 2: Building a Microsoft Access Desktop Application
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Figure 9-7. The dialog box that asks you to confirm an action query.
Updating Multiple Fields
When you create an update query, you aren t limited to changing a single field at a time. You can ask Access to update any or all of the fields in the record by including them in the design grid and then specifying an update formula. Before Access updates a record in the underlying table or query, it makes a copy of the original record. Access applies the formulas you specify using the values in the original record and places the result in the updated copy. It then updates your database by writing the updated copy to your table. Because updates are made to the copy before updating the table, you can, for example, swap the values in a field named A and a field named B by specifying an Update To setting of [B] for the A field and an Update To setting of [A] for the B field. If Access were making changes directly to the original record, you d need to use a third field to swap values because the first assignment of B to A would destroy the original value of A. If you remember from 7, we also discussed the possibility of reducing the highest daily rate charged for a room to $90. If you do that, you must also update the WeeklyRate value to make sure it doesn t exceed six times the new daily rate. First, build a query to find all rows that have a value in the DailyRate field that exceeds the new maximum. As before, start a query on tblFacilityRooms and include the FacilityID, RoomNumber, DailyRate, and WeeklyRate fields. Place the criterion >90 under the DailyRate field. Your query should look like Figure 9-8. If you run this query, you ll find 10 rows that meet this criterion in the sample database.
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Figure 9-8. A query to find daily rates greater than $90.
Part 1: Part Title
Modifying Data with Action Queries Now comes the tricky part. Change your query to an update query and enter 90 on the Update To row under DailyRate. You might be tempted to set Update To under WeeklyRate to [DailyRate]*6 again, but you would be wrong. The reference to [DailyRate] gets you the original value of that field in each row before it gets updated to the value 90. You know you re going to set DailyRate in rows that qualify to 90, so enter the constant 540 or the expression (90 * 6) in the Update To line under WeeklyRate. Your update query should now look like Figure 9-9. Tip
Performing multiple updates with expressions that reference table fields If you want to increase (or decrease) DailyRate by some percentage, then you should repeat the calculation for the new DailyRate value and multiply by 6 to calculate the new WeeklyRate. For example, if you want to increase the rate by 10 percent, your expression in Update To for the DailyRate field is
CCur(Round([DailyRate] * 1.1, 0))
Then your expression under WeeklyRate should be
CCur(Round([DailyRate] * 1.1), 0)) * 6
Remember, DailyRate in any expression references the old value in the row before it is updated.
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Figure 9-9. A query to find daily rates greater than $90.
You can find this query saved as qxmplUpdateDailyWeekly in the sample database.
Creating an Update Query Using Multiple Tables or Queries
The target of an update query should generally be one table, but you might need to update a table based on criteria you apply to a related query or table. Consider the tblContacts table in the LawTrack Contacts sample application. The table contains an Inactive field that you can set to Yes to remove that contact from most displays without removing the row from the database. Although you can edit each contact individually and choose to mark them inactive, 343
Part 2: Building a Microsoft Access Desktop Application
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Part 1: Part Title
Microsoft Office Access 2003 Inside Out you occasionally might want to run an update query that automatically sets this flag when the contact hasn t had any activity for a long time. You studied how to solve a complex unmatched problem in 8, Building Complex Queries. You need to apply a similar concept to this problem find the contacts who have activity since a certain date of interest and then use an outer join to identify those who have no activity so that you can mark them inactive. The sample database contains contact events from November 2, 2002, through April 25, 2003. If you were using this data actively, you would be entering new contact events every day, but this sample data is static. Let s assume that today is October 1, 2003, and you want to flag any contact who hasn t had any event in the last six months. First, you need to find out who hasn t contacted you since April 1, 2003. Start by opening the ContactsDataCopy.mdb sample database. Start a query on tblContactEvents and include the fields ContactID and ContactDateTime in the query grid. Under ContactDateTime, enter a criterion of >=#4/1/2003#. Your query should look like Figure 9-10.
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