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We can create Cascade options on the FOREIGN KEY constraint to cause the data deletions or updates in the parent table to cascade to the child table Cascade options are defined as part of the FOREIGN KEY definition For example, if we consider our Employees and Address tables created earlier, we may decide that if an Employee is deleted, we also want to delete the row in the Address table EXAM TIP If we want to automate the deletions of records in FOREIGN KEY tables when a record in the PRIMARY KEY is deleted, we can define an On Delete Cascade option to have deletions cascade We would define the cascade option on the FOREIGN KEY constraint We could alter the Employee table with the following script to implement cascading deletes from the Employees table to the Address table First, we would drop the existing FK constraint and then re-create it with the ON DELETE CASCADE clause
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USE 3; Go Alter TABLE dboEmployee Drop Constraint FK_Employee_Address; GO Alter TABLE dboEmployee ADD CONSTRAINT FK_Employee_Address FOREIGN KEY(AddressID) REFERENCES dboAddress(AddressID) ON DELETE CASCADE
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NOTE While this example does show us how to implement a cascading delete, it wouldn t be acceptable if we had multiple employees (such as a husband and wife) tied to the same address If the husband was deleted, the wife s address would also be deleted This is an excellent example of why cascading delete isn t the default If we consider our Orders and OrderDetails tables from the previous example, the OrderDetails table could be defined with an On Delete Cascade option on the FOREIGN KEY constraint Now, if an order is deleted, all order detail records for this order would also be deleted We can also create an On Update Cascade, which causes updates in the PRIMARY KEY table to cascade down to the FOREIGN KEY table This affects the related fields (PRIMARY KEY and FOREIGN KEY fields) only and since we rarely change the PRIMARY KEY data it wouldn t be used as often Consider this scenario: We have a PK of three digits on our Address table The maximum number of addresses we can have is 1000 (0 to 999) The company grows and we decide to modify the PK of the address table to include the first three initials of the city followed by a six-digit number If we modified the existing PRIMARY KEYs in the Address table, the Employee addresses would now be orphaned with FKs in the Employees table pointing to nonexistent PKs Instead, we could use On Update Cascade When we modified the PK in the Address table, the change to the PK would be cascaded to the Employee table
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Let s consider one more example of using PRIMARY and FOREIGN KEYs to help us ensure that only valid data is entered Let s say we have customers only from the United States We know that valid entries are limited to the 50 states We can create a table (named State) that contains one char(2) column (named State) set as the PK We could then enter the two-letter abbreviation of all 50 states into this table, and create an FK from the Address table to this State table Figure 3-4 shows a database diagram of these two tables
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Figure 3-4 Database diagram of Address and State tables
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Now, whenever the state abbreviation is entered into the Address table, SQL Server compares the entry to the 50 valid entries in the State table If someone makes a typo
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(such as VS instead of VA for Virginia), the FK constraint will prevent the data entry until it s corrected Of course, if the user enters MA instead of VA for Virginia, the database will allow it since MA is valid for Massachusetts We can check to ensure the data is valid, but we can t make sure it s correct EXAM TIP An FK constraint can be very effective in ensuring only valid data is entered If the data doesn t exist in the child record, the entry fails Compared with other methods, an FK constraint is often the most effective because it looks at the data before the data is entered In contrast, DML triggers work on data after the data has been entered Triggers can roll back the data entry, but that requires double work since the data is entered, and then undone The following exercise will demonstrate using a constraint for data validation Exercise 31: Enforce Integrity with an FK 1 Open SSMS if it isn t open already and create a New Query window 2 If you didn t run the scripts described previously in this chapter, run them now: a Run the script to create the 3 database b USE Master CREATE DATABASE 3;Run the script to create the Employee table
USE 3; GO CREATE TABLE dboEmployee ( EmployeeID int IDENTITY(100,1) NOT NULL, LastName varchar(35) NULL, FirstName varchar(35) NULL, CONSTRAINT [PK_Employee_EmployeeID] PRIMARY KEY CLUSTERED ( EmployeeID ) );
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