create qr code vb.net Part 4: Designing an Access Project in .NET

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Part 4: Designing an Access Project
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Part 4: Designing an Access Project
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Figure 17-18. Creating a new check constraint on the Contacts table.
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A constraint expression can be any valid SQL expression that evaluates to true or false. One of the rules you need to enforce for the Contacts table is that no contact can have a commission percent greater than 90%. To define this rule, you must enter an expression in the Constraint expression box that evaluates to false if anyone enters a commission percent greater than 90% for a contact. To create the commission percent expression, type the following in the Constraint expression box: CommissionPercent <= 0.9. When you attempt to save a value in CommissionPercent and this expression evaluates to false (you entered a commission percent greater than 90%), Access displays the validation message, and the row will not be saved. You can use many other types of expressions to validate the data that is being added or updated. For now, let s finish creating the commission percent check constraint and make sure it works. Now that you have a valid expression in the Constraint expression box, you can change the name of the check constraint to something more meaningful. Click in the Constraint name field and enter CK_Contacts_CommissionPercent as the constraint name. This is a useful name because when you look at objects in the database, CK tells you that this object is a check constraint, Contacts tells you that it is applied to the Contacts table, and CommissionPercent tells you which column is being validated.
You might notice that if you try to move the focus out of the Constraint expression box while it contains a partially complete or incorrect expression, Access immediately generates an error stating that the expression could not be validated. You can click Yes to keep working on the expression or you can click No to move the focus away from the Constraint expression box. If the Constraint expression box is blank, Access offers you the option to delete the constraint. In this case, click No to continue working on the constraint. Be careful: Access cannot save the constraint unless the expression is valid.
Note
Part 1: Part Title
Building Tables in an Access Project Because all check constraints are evaluated only when the entire row is updated, it is important to make sure that the validation text is descriptive enough to tell the user where the problem is so that it can be fixed. In the Validation Text box, type You cannot specify a commission greater than 90%. This will alert the user that the commission percent value is too great if the validation of the constraint fails. If you want to make sure that all existing contacts meet the commission percent constraint, leave the Check existing data on creation option selected. When you select this option, SQL Server will not save the constraint if any existing rows fail. The Enforce constraint for replication option allows you to apply the constraint to any copies of the database that you distribute for replication. This option is useful if you ever plan to replicate your database. For now, you can leave this option blank. In order for the constraint to be applied whenever data is inserted or updated, you must leave the Enforce constraint for INSERTs and UPDATEs option selected. When you are done creating the commission percent check constraint, it should look like Figure 17-19. If you need to add additional check constraints, click the New button. You can also delete a check constraint by choosing it from the Selected constraint drop-down list and then clicking the Delete button.
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Figure 17-19. Completing the commission percent check constraint on the Contacts table.
Part 4: Designing an Access Project
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When you are finished creating check constraints, close the Properties window and save the table. When you save the table, the check constraints are saved as well. Now open the table in Datasheet view and see if the check constraint is working correctly. Tab to the CommissionPercent column in an existing row and try entering a commission percent of 100%. Press the Down Arrow key to move to a different record, and you should see the error message in Figure 17-20 on the next page.
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