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consequences of making the MOVIE table a mandatory child of the MPAA_RATING table. If the Motion Picture Association of America (MPAA) created a new rating code, you would not be able to add it to the MPAA_RATING table until you had a movie to add to the MOVIE table. Likewise, you would not be able to delete the last row in the MOVIE table that matched any particular rating code without deleting the corresponding MPAA_RATING table row. These awkward restrictions are likely the reason that relational databases do not provide direct support for mandatory children in one-to-many relationships. Relationships are implemented using matching columns in the two participating tables. On the ERD, the underlined column(s) in each table with the notation <pk> to their right form the primary key, which is a column or a set of columns that uniquely identi es each row in a table. Each table may have only one primary key. However, a primary key may be composed of multiple columns if that is what it takes to form a unique key. Primary keys are very important because they are the foundation for relationships. Whenever a primary key is used in another table to establish a relationship, it is called a foreign key. In Figure 1-2, note the foreign key columns in the MOVIE table that establish relationships with the MOVIE_GENRE and MPAA_RATING tables, which are noted with <fk1> and <fk2> to the right of the foreign key column names. The LANGUAGE_CODE column is also noted as a foreign key ( <fk3> ) but the LANGUAGE table and its relationship with the MOVIE table have been omitted from Figure 1-2. Also notice that the primary key of the MOVIE table appears in the child table MOVIE_COPY as a foreign key to establish the relationship between those two tables. Primary keys and foreign keys are the fundamental building blocks of the relational model because they establish relationships and provide the ability to link data from multiple tables when required. You must understand this concept in order to understand how relational databases work.
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A constraint is a rule placed on a database object (typically a table or column) that restricts the allowable data values for that database object in some way. Once in place, constraints are automatically enforced by the DBMS and cannot be circumvented unless an authorized person disables or deletes (drops) the constraint. Each constraint is assigned a unique name to permit it to be referenced in error messages and subsequent database commands. It is a good habit for database designers to supply the constraint names because names generated automatically by the database are not very descriptive. However, I did not supply constraint names in the sample database included in this book because, unfortunately, not all RDBMS products available today support named constraints.
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There are several types of database constraints:
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NOT NULL constraint May be placed on a database column to prevent the use of null values. A null value is a special way in which the RDBMS handles a column value to indicate that the value for that column in that row is unknown. A null is not the same as a blank, an empty string, or a zero it is indeed a special value that is not equal to anything else. Null values are discussed in more detail in 3. Primary key constraint De ned on the primary key column(s) of a table to guarantee that the primary key values are always unique within the table. When de ned on multiple columns of a table, it is the combination of all column values that must be unique within the table a column that is only part of a primary key may have duplicate values in the table. Primary key constraints are nearly always implemented by the RDBMS using an index, which is a special type of database object that permits fast searches of column values. As new rows are inserted into the table, the RDBMS automatically searches the index to make sure the value for the primary key of the new row is not already in use in the table, rejecting the insert request if it is. Indexes can be searched much faster than tables; therefore, the index on the primary key is essential in tables of any size so that the search for duplicate keys on every insert doesn t create a performance bottleneck. An additional characteristic of primary key constraints is that they can only be de ned on columns that also have a NOT NULL constraint de ned. Unique constraint De ned on a column or set of columns in a table that must contain unique values within the table. As with a primary key constraint, the RDBMS almost always uses an index as a vehicle to ef ciently enforce the constraint. However, unlike primary key constraints, a table may have multiple unique constraints de ned on it, and columns that participate in a unique constraint may (in most RDBMSs) contain null values. Referential constraint (sometimes called a referential integrity constraint) A constraint that enforces a relationship between two tables in a relational database. By enforces I mean that the RDBMS automatically checks to ensure that each foreign key value always has a corresponding primary key value in the parent table. In the MOVIE table (see Figure 1-1), the RDBMS would prevent me from inserting a movie with an MPAA_ RATING_CODE of M because M is no longer a valid MPAA_RATING_ CODE and therefore does not appear as a primary key value in the MPAA_ RATING table. Conversely, the RDBMS would prevent me from deleting the row in the MPAA_RATING table with the primary key value of PG-13 because that primary key value is in use as a foreign key value in at least one
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