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On the surface, there s nothing wrong with this INSERT statement. In fact, some SQL implementations will successfully add the row. The database will show that George Smith works in office number 31, even though no office number 31 is listed in the OFFICES table. The newly inserted row clearly breaks the parent/child relationship between the OFFICES and SALESREPS tables. In fact, the office number in the INSERT statement is probably an error the user may have intended office number 11, 21, or 13.

Figure 4-5

FIGURE 11-1

The CD_TITLE_ID column in the CD_TITLES table is configured with a PRIMARY KEY constraint, as is the PUBLISHER_ID column in the CD_PUBLISHERS table. Both these columns are shaded in the illustration. Notice that the CD_TITLES table contains a column named PUBLISHER_ID. This column includes values from the PUBLISHER_ID column of the CD_PUBLISHERS table. In fact, the PUBLISHER_ID values in the CD_TITLES table should include only values that come from the PUBLISHER_ID column in the CD_PUBLISHERS table. You should not be able to insert a row into CD_TITLES if the PUBLISHER_ID value is not listed in the CD_PUBLISHERS table. At the same time, if you alter or delete a PUBLISHER_ID value in the CD_PUBLISHERS table, you should be able to predict the outcome of your action if those same values exist in the CD_TITLES table. Under no circumstances would you want to delete a publisher and leave PUBLISHER_ID values in the CD_TITLES table that reference a publisher that no longer exists. These results can be achieved by using a FOREIGN KEY constraint. A FOREIGN KEY constraint enforces referential integrity between two tables by ensuring that no action is taken on either table that adversely affects the data protected by the constraint. In the tables shown in Figure 4-5, the FOREIGN KEY constraint must be configured on the PUBLISHER_ID column of the CD_TITLES table. The FOREIGN KEY constraint restricts the values in that column to the values of a candidate key (usually the primary key) in the related table. Only valid data values are permitted in the FOREIGN KEY column or columns.

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Part III:

It seems clear that every legal value in the REP_OFFICE column should be forced to match some value that appears in the OFFICE column. This rule is known as a referential integrity constraint. It ensures the integrity of the parent/child relationships created by foreign keys and primary keys. Referential integrity has been a key part of the relational model since it was first proposed by E. F. Codd. However, referential integrity constraints were not included in IBM s prototype System/RDBMS, nor in early releases of DB2 or SQL/DS. IBM added referential integrity support to DB2 in 1989, and referential integrity was added to the SQL standard after the release of the initial (SQL1) version. Most DBMS vendors today support referential integrity constraints.

The table that contains the foreign key is the referencing table. The table that is being referenced by the foreign key is the referenced table. Likewise, the column or columns that make up the foreign key in the referencing table are referred to as the referencing columns. The columns being referenced by the foreign key are the referenced columns.

Four types of database updates can corrupt the referential integrity of the parent/child relationships in a database Using the OFFICES and SALESREPS tables in Figure 11-1 as illustrations, the four update situations are the following: Inserting a new child row When an INSERT statement adds a new row to the child (SALESREPS) table, its foreign key (REP_OFFICE) value must match one of the primary key (OFFICE) values in the parent table (OFFICES) If the foreign key value does not match any primary key, inserting the row will corrupt the database, because there will be a child without a parent (an orphan) Note that inserting a row in the parent table never poses a problem; it simply becomes a parent without any children Updating the foreign key in a child row This is a different form of the previous problem.

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