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This statement defines a FOREIGN KEY constraint on the PUBLISHER_ID column. Notice that, in order to add a column constraint, all you have to do is add the REFERENCES keyword and the name of the referenced table. Also notice that the foreign key contains the same number of columns as the primary key in the referenced table, and the referenced and referencing columns are the same data type. Remember, if you re not referencing the primary key in the referenced table, you must also include the name of the column or columns for example, REFERENCES CD_PUBLISHERS (PUBLISHER_ID).

Part VI:

It s easiest to explain the basics of stored procedures through an example. Consider the process of adding a customer to the sample database. Here are the steps that may be involved: 1. Obtain the customer number, name, credit limit, and target sales amount for the customer, as well as the assigned salesperson and office. 2. Add a row to the customer table containing the customer s data. 3. Update the row for the assigned salesperson, raising the quota target by the specified amount. 4. Update the row for the office, raising the sales target by the specified amount. 5. Commit the changes to the database, if all previous statements were successful. Without a stored procedure capability, here is a SQL statement sequence that does this work for XYZ Corporation, new customer number 2137, with a credit limit of $30,000 and first-year target sales of $50,000 to be assigned to Paul Cruz (employee #103) of the Chicago office:

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Before you can create a foreign key on a table, the referenced table must already exist and a UNIQUE or PRIMARY KEY constraint must be defined for that table.

INSERT INTO CUSTOMERS (CUST_NUM, COMPANY, CUST_REP, CREDIT_LIMIT) VALUES (2137, 'XYZ Corporation', 103, 30000.00); UPDATE SALESREPS SET QUOTA = QUOTA + 50000.00 WHERE EMPL_NUM = 103; UPDATE OFFICES SET TARGET = TARGET + 50000.00 WHERE CITY = 'Chicago'; COMMIT;

In the next example, I create a foreign key that is a table constraint. Unlike the previous example, I include the name of the referenced column in this constraint definition, even though it isn t necessary:

With a stored procedure, all of this work can be embedded into a single defined SQL routine. Figure 20-1 shows a stored procedure for this task, expressed in Oracle s PL/SQL stored procedure dialect. The procedure is named ADD_CUST, and it accepts six parameters the customer name, number, credit limit, and target sales, the employee number of the assigned salesperson, and the city where the assigned sales office is located. Once this procedure has been created in the database, a statement like this one:

ADD_CUST('XYZ Corporation', 2137, 30000.00, 50000.00, 103, 'Chicago');

CREATE TABLE CD_TITLES ( CD_TITLE_ID INT, CD_TITLE VARCHAR(60), PUBLISHER_ID INT, CONSTRAINT FK_PUBLISHER_ID FOREIGN KEY (PUBLISHER_ID) REFERENCES CD_PUBLISHERS (PUBLISHER_ID) );

calls the stored procedure and passes it the six specified values as its parameters. The DBMS executes the stored procedure, carrying out each SQL statement in the procedure definition one by one. If the ADD_CUST procedure completes its execution successfully, a committed transaction has been carried out within the DBMS. If not, the returned error code and message indicates what went wrong.

20:

The last two lines of code are the constraint definition The name of the constraint, FK_PUBLISHER_ID, follows the CONSTRAINT keyword Constraint names aren t necessary because the DBMS will assign a system-generated name if one is not supplied However, it s a good practice to supply your own because constraint names often appear in error messages when SQL statements attempt to violate constraints, and names you supply will be easier to recognize than ones the DBMS supplied for you Following the constraint name, the FOREIGN KEY keywords indicate the type of constraint, which is followed by the referencing column name, PUBLISHER_ID This is the name of the column on which the constraint is being placed If there were multiple column names, they would be separated by commas.

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