Design Data Integrity in VB.NET

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Design Data Integrity
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must be linked to columns that either have a PRIMARY KEY or UNIQUE constraint applied. You can use FOREIGN KEY constraints to enforce referential integrity. When referential integrity is enforced, data in the foreign table is protected from deletion if that deletion orphans data in the referencing table. For example, in the AdventureWorks database there are two tables, Sales.SalesPerson and Sales.SalesOrderHeader, that are linked using a FOREIGN KEY constraint. The SalesPersonID column of the Sales.SalesOrderHeader table is linked using a FOREIGN KEY constraint to the SalesPersonID column of the Sales.SalesPerson table. Referential integrity ensures that a salesperson s row cannot be deleted from the Sales.SalesPerson table as long as any row in the Sales.SalesOrderHeader table makes reference to that salesperson s ID. When SQL Server enforces referential integrity, you can delete a row only from a table that is the target of a FOREIGN KEY constraint if the data in that row is not referenced in the table to which you have applied the FOREIGN KEY constraint.
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Limit on number of FOREIGN KEY constraints
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Although SQL Server does not have any default limit on the number of FOREIGN KEY constraints that you can apply to a table, the number of constraints is limited by hardware configuration and database design. Microsoft recommends that no table contain more than 253 FOREIGN KEY constraints and that no single table should be referenced by more than 253 FOREIGN KEY constraints.
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To create a FOREIGN KEY constraint, perform the following steps: 1. Open SSMS, and connect to the database instance that hosts the table that has the column to which you want to apply the FOREIGN KEY constraint. 2. Expand the Databases folder to locate the database that hosts the table that has the column to which you want to apply the FOREIGN KEY constraint. 3. Expand the appropriate database, and locate the table that has the column to which you want to apply the FOREIGN KEY constraint. 4. Right-click the table and choose Modify. 5. From the Table Designer menu, choose Relationships. 6. In the Foreign-Key Relationships dialog box, click Add. 7. Click Tables And Columns Specification in the grid to the right, and then click the ellipses (...) to the right of the property. 8. In the Tables And Columns dialog box, in the Primary Key Table drop-down list, select the table that will be on the primary or unique key side of the relationship.
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Lesson 2: Making Implicit Constraints Explicit
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9. In the grid beneath the Primary Key Table text box, select the column that will be used as the table s primary key. 10. In the adjacent grid cell, select the column in the table that will be using the foreign key, as shown in Figure 8-5.
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Figure 8-5
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Configuring a FOREIGN KEY constraint.
11. Click OK to create the relationship, and then click Close to close the Foreign Key Relationships dialog box.
Real World
Orin Thomas When I first got my driver s license, the person entering the data to be printed on the license kept misspelling the name of the suburb in which I lived. After the third error, I decided to put up with the mistake and hope that when I renewed my license ten years later the person responsible for entering the data would know how to spell. This would not have been a problem if the license database had some sort of FOREIGN KEY constraint linking the suburb I lived in to a list of all localities within my state as a way of enforcing data integrity. If a FOREIGN KEY had been in place, only an approved spelling of my suburb s name would have been able to have been used on my license. Alas, the technology wasn t sophisticated enough in those days, and I often had to explain that some of the information on my primary form of identification was spelled incorrectly.
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