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CREATE TABLE dbovehicle( objid int IDENTITY(1,1) NOT NULL, make varchar(50) NOT NULL, model varchar(50)NOT NULL, year char(4) NOT NULL, employee_objid int NOT NULL, CONSTRAINT PK_vehicle PRIMARY KEY (objid),
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CONSTRAINT FK_vehicle_employee FOREIGN KEY(employee_objid) REFERENCES employee (objid)
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Once your primary keys are in place, the creation of the foreign keys is academic You simply create the appropriate columns on the referencing table and add the foreign key As stated in 2, if your design requires it, the same column in a table can be in both the primary key and a foreign key When you create foreign keys, you can also specify what to do if an update or delete is issued on the parent table By default, if you attempt to delete a record in the parent table, the delete will fail because it would result in orphaned rows in the referencing table An orphaned row is a row that exists in a child table that has no corresponding parent This can cause problems in some data models In our employee and vehicle tables, a NULL in the vehicle table means that the vehicle has not been assigned to an employee However, consider a table that stores orders and order details; in this case, an orphaned record in the order detail table would be useless You would have no idea which order the detail line belonged to Instead of allowing a delete to fail, you have options First, you can have the delete operation cascade, meaning that SQL Server will delete all the child rows along with the parent row you are deleting Be very careful when using this option If you have several levels of relationships with cascading delete enabled, you could wipe out a large chunk of data by issuing a delete on a single record Your second option is to have SQL Server set the foreign key column to NULL in the referencing table This option creates orphaned records, as discussed Third, you can have SQL Server set the foreign key column back to the default value of the column, if it has one Similar options are also available if you try to update the primary key value itself Again, SQL Server can either (1) cascade the update so that the child rows still point to the correct parent rows with the new key, (2) set the foreign key to NULL, or (3) set the foreign key back to its default value
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Changing the values of primary keys isn t something we recommend you do often, but in some situations you may find yourself needing to do just that If you find yourself in that situation often, you might consider setting up an update rule on your foreign keys
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SQL Server contains several types of constraints to enforce data integrity Constraints, as the name implies, are used to constrain the values that can be entered into columns We have talked about two of the constraints in SQL Server: primary keys and foreign keys Primary keys constrain the data so that duplicates and NULLs cannot exist in the columns, and foreign keys ensure that the entered value exists in the referenced table There are several other constraints you can implement to ensure data integrity or enforce business rules
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Unique constraints are similar to primary keys; they ensure that no duplicates exist in a column or collection of columns They are configured on columns that do not participate in the primary key How does a unique constraint differ from a primary key From a technical standpoint, the only difference is that a unique constraint allows you to enter NULL values; however, because the values must be unique, you can enter only one NULL value for the entire column When we talked about identifying primary keys, we talked about candidate keys Because candidate keys should also be able to uniquely identify the row, you should probably place unique constraints on your candidate keys You add a unique constraint in much the same way as you add a foreign key, using a constraint statement such as
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CONSTRAINT UNQ_vehicle_vin UNIQUE NONCLUSTERED (vin_number)
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