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If the foreign key (REP_OFFICE) is modified by an UPDATE statement, the new value must match a primary key (OFFICE) value in the parent (OFFICES) table Otherwise, the updated row will be an orphan Deleting a parent row If a row of the parent table (OFFICES) that has one or more children (in the SALESREPS table) is deleted, the child rows will become orphans The foreign key (REP_OFFICE) values in these rows will no longer match any primary key (OFFICE) value in the parent table Note that deleting a row from the child table never poses a problem; the parent of this row simply has one less child after the deletion Updating the primary key in a parent row This is a different form of the previous problem.

When creating a FOREIGN KEY constraint, you must follow several guidelines:

If the primary key (OFFICE) of a row in the parent table (OFFICES) is modified, all of the current children of that row become orphans because their foreign keys no longer match a primary key value The referential integrity features of the ANSI/ISO SQL standard handle each of these four situations The first problem (INSERT into the child table) is handled by checking the values of the foreign key columns before the INSERT statement is permitted If they don t match a primary key value, the INSERT statement is rejected with an error message In Figure 11-1, this means that before a new salesperson can be added to the SALESREPS table, the office to which the salesperson is assigned must already be in the OFFICES table As you can see, this restriction makes sense in the sample database..

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The referenced columns must be defined with either a UNIQUE or PRIMARY KEY constraint on them. As you might guess, the primary key is most commonly used for the referenced columns. A FOREIGN KEY constraint can be created as a table constraint or column constraint. If you create the foreign key as a column constraint, you can include only one column. If you create the foreign key as a table constraint, you can include one or more columns. The foreign key in the referencing table must include the same number of columns that are being referenced, and the referencing columns must each be configured with the same data types as their referenced counterparts. However, the referencing columns do not have to have the same names as the referenced columns. If you don t specify the referenced columns when you define a FOREIGN KEY constraint, then the columns defined in the primary key of the referenced table are used as the referenced columns.

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In its original form, SQL was not envisioned as a complete programming language. It was designed and implemented as a language for expressing database operations creating database structures, entering data into the database, updating database data and especially for expressing database queries and retrieving the answers. SQL could be used interactively by typing SQL statements at a keyboard, one by one. In this case, the sequence of database operations was determined by the human user. SQL could also be embedded within another programming language, such as COBOL or C. In this case, the sequence of database operations was determined by the flow of control within the COBOL or C program. With stored procedural SQL, the SQL language is extended with several capabilities normally associated with programming languages. Sequences of extended SQL statements are grouped together to form SQL programs or procedures. (For simplicity, we refer to stored procedures, functions, and triggers collectively as SQL procedures.) The specifics vary from one implementation to another, but generally, these capabilities are provided: Conditional execution An IF THEN ELSE structure allows a SQL procedure to test a condition and to carry out different operations depending on the result. Looping A WHILE or FOR loop or similar structure allows a sequence of SQL operations to be performed repeatedly, until some terminating condition is met. Some implementations provide a special cursor-based looping structure to process each row of query results.

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Block structure A sequence of SQL statements can be grouped into a single block and used in other flow-of-control constructs as if the statement block were a single statement Named variables A SQL procedure may store a value that it has calculated, retrieved from the database, or derived in some other way into a program variable, and later retrieve the stored value for use in subsequent calculations Named procedures A sequence of SQL statements may be grouped together, given a name, and assigned formal input and output parameters, like a subroutine or function in a conventional programming language Once defined in this way, the procedure may be called by name, passing it appropriate values for its input parameters If the procedure is a function returning a value, it may be used in SQL value expressions Collectively, the structures that implement these capabilities form a stored procedural language (SPL).

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