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PART I
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FIGURE 4-9
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A parent/child relationship in a relational database
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Foreign Keys
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A column in one table whose value matches the primary key in some other table is called a foreign key. In Figure 4-9, the REP_OFFICE column is a foreign key for the OFFICES table. Although REP_OFFICE is a column in the SALESREPS table, the values that this column contains are office numbers. They match values in the OFFICE column, which is the primary key for the OFFICES table. Together, a primary key and a foreign key create a parent/child relationship between the tables that contain them, just like the parent/child relationships in a hierarchical database. Just as a combination of columns can serve as the primary key of a table, a foreign key can also be a combination of columns. In fact, the foreign key will always be a compound (multicolumn) key when it references a table with a compound primary key. Obviously, the number of columns and the data types of the columns in the foreign key and the primary key must be identical to one another. A table can contain more than one foreign key if it is related to more than one other table. Figure 4-10 shows the three foreign keys in the ORDERS table of the sample database: The CUST column is a foreign key for the CUSTOMERS table, relating each order to the customer who placed it. The REP column is a foreign key for the SALESREPS table, relating each order to the salesperson who took it. The MFR and PRODUCT columns together are a composite foreign key for the PRODUCTS table, relating each order to the product being ordered. The multiple parent/child relationships created by the three foreign keys in the ORDERS table may seem familiar to you, and they should. They are precisely the same relationships as those in the network database of Figure 4-4. As the example shows, the relational data model has all of the power of the network model to express complex relationships.
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Relational Databases
Foreign keys are a fundamental part of the relational model because they create relationships among tables in the database. As with primary keys, foreign key support was missing from early relational database management systems. They were added to DB2 Version 2, were subsequently added to the ANSI/ISO standard, and now appear in all of the major commercial products.
PART I
Codd s 12 Rules for Relational Databases*
As the relational database model started to become very popular in the mid-1980s, every DBMS vendor scrambled to describe their product as relational. Some of these products had only a SQL-like query language layered on top of an underlying network or hierarchical database. Some of them implemented only a very rudimentary table structure and no query language at all. Soon the question of What is a true relational database became a topic of debate, and DBMS vendors began claiming that their products were more relational than the competition. In 1985, Ted Codd, whose seminal technical article 15 years earlier had defined the relational data model, addressed this question in Computerworld, one of the leading trade publications. In his two-part article, entitled Is Your DBMS Really Relational (October 14, 1985) and Does Your DBMS Run By the Rules (October 21, 1985), Codd presented 12 rules that a database must obey if it is to be considered truly relational: 1. Information rule. All information in a relational database is represented explicitly at the logical level and in exactly one way by values in tables. 2. Guaranteed access rule. Each and every datum (atomic value) in a relational database is guaranteed to be logically accessible by resorting to a combination of table name, primary key value, and column name. 3. Systematic treatment of NULL values. NULL values (distinct from an empty character string or a string of blank characters and distinct from zero or any other number) are supported in a fully relational DBMS for representing missing information and inapplicable information in a systematic way, independent of the data type. 4. Dynamic online catalog based on the relational model. The database description is represented at the logical level in the same way as ordinary data, so that authorized users can apply the same relational language to its interrogation as they apply to the regular data. 5. Comprehensive data sublanguage rule. A relational system may support several languages and various modes of terminal use (for example, the fill-in-the-blanks mode). However, there must be at least one language whose statements are expressible, per some well-defined syntax, as character strings, and that is comprehensive in supporting all of the following items: Data definition View definition Data manipulation (interactive and by program) Integrity constraints Authorization Transaction boundaries (begin, commit, and rollback)
Part I:
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