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Adding NOT NULL, Unique, and Referential Constraints
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In 3, Try This 3-1 and Try This 3-2, you created several tables that you added to the INVENTORY database (or the CD_INVENTORY schema). In this Try This exercise, you will add a number of constraints to the tables and create new tables that are also defined with constraints. However, rather than use the ALTER TABLE statement to modify the tables that you already created, you will be recreating those tables. The advantage to this is that you ll be able to see the complete table definition as it relates to the updated data model, shown in Figure 4-7. The data model incorporates a few more elements than you have seen before. It identifies tables, columns within those tables, data types for those columns, constraints, and relationships between tables. You should already be familiar with how tables, columns, and data types are represented, so let s take a look at constraints and relationships:
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The columns included in the primary key are in the top section of the table, and the other columns lie in the bottom section. For example, in the COMPACT_DISCS table, the COMPACT_DISC_ID column is the primary key. In some cases, as in the COMPACT_ DISC_TYPES table, all columns are included in the primary key. Each foreign key is represented by an [FK]. Defaults, UNIQUE constraints, and NOT NULL constraints are identified with each applicable column. Relationships, as defined by foreign keys, are represented by lines that connect the foreign key in one table to the candidate key (usually the primary key) in another table.
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You ll find this data model useful not only for this exercise, but for other Try This exercises in the book, all of which will continue to build upon or use the INVENTORY database. You can also download the Try_This_04.txt#file, which contains the SQL statements used in this exercise.
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ARTIST_CDS ARTIST_ID INT [FK] COMPACT_DISC_ID INT [FK]
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ARTISTS ARTIST_ID INT ARTIST_NAME VARCHAR(60) NOT NULL PLACE_OF_BIRTH VARCHAR(60) DEFAULT 'Unknown' NOT NULL
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COMPACT_DISCS COMPACT_DISC_ID INT CD_TITLE VARCHAR(60) NOT NULL LABEL_ID INT NOT NULL [FK]
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COMPACT_DISC_TYPES COMPACT_DISC_ID INT [FK] MUSIC_TYPE_ID INT [FK]
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MUSIC_TYPES TYPE_ID INT TYPE_NAME VARCHAR(20) NOT NULL UNIQUE
CD_LABELS LABEL_ID INT COMPANY_NAME VARCHAR(60) DEFAULT Independent' NOT NULL
Figure 4-7
Data model for the INVENTORY database
NOTE
Data models come in many varieties. The model I use here is specific to the needs of the book. You ll find in the real world that the models will differ from what you see here. For example, relationships between tables might be represented differently, and column definition information might not be quite as extensive.
Step by Step
1. Open the client application for your RDBMS and connect to the INVENTORY database. 2. You first need to drop the four tables (COMPACT_DISCS, COMPACT_DISC_TYPES,
MUSIC_TYPES, and CD_LABELS) that you already created. Enter and execute the following SQL statements:
DROP DROP DROP DROP TABLE TABLE TABLE TABLE COMPACT_DISCS COMPACT_DISC_TYPES MUSIC_TYPES CD_LABELS CASCADE; CASCADE; CASCADE; CASCADE;
4:
Enforcing Data Integrity
NOTE
If you created either the ARTISTS table or the ARTIST_CDS table when trying out examples or experimenting with CREATE TABLE statements, be sure to drop those as well.
NOTE
The CASCADE option is not supported by SQL Server, and in Oracle must be written as CASCADE CONSTRAINTS.
Now you can begin to recreate these tables and create new ones. You should create the tables in the order outlined in this exercise because the tables referenced in foreign keys will have to exist with primary keys created before you can create the foreign keys. Be sure to refer to the data model in Figure 4-7 for details about each table that you create.
3. The first table that you re going to create is the MUSIC_TYPES table. It contains two
columns: TYPE_ID and TYPE_NAME. You ll configure the TYPE_ID column as the primary key, and you ll configure a UNIQUE constraint and NOT NULL constraint on the TYPE_NAME column. Enter and execute the following SQL statement:
CREATE TABLE ( TYPE_ID TYPE_NAME CONSTRAINT CONSTRAINT MUSIC_TYPES INT, VARCHAR(20) NOT NULL, UN_TYPE_NAME UNIQUE (TYPE_NAME), PK_MUSIC_TYPES PRIMARY KEY (TYPE_ID) );
4. The next table that you ll create is the CD_LABELS table. The table includes the LABEL_
ID column, which will be defined as the primary key, and the COMPANY_NAME column, which will be defined with a default and the NOT NULL constraint. Enter and execute the following SQL statement:
CREATE TABLE CD_LABELS ( LABEL_ID INT, COMPANY_NAME VARCHAR(60) DEFAULT 'Independent' NOT NULL, CONSTRAINT PK_CD_LABELS PRIMARY KEY (LABEL_ID) ); 5. Now that you ve created the CD_LABELS table, you can create the COMPACT_
DISCS table. The COMPACT_DISCS table contains a foreign key that references the CD_LABELS table. This is why you created CD_LABELS first. Enter and execute the following SQL statement:
CREATE TABLE COMPACT_DISCS ( COMPACT_DISC_ID INT, CD_TITLE VARCHAR(60) NOT NULL, LABEL_ID INT NOT NULL, CONSTRAINT PK_COMPACT_DISCS PRIMARY KEY (COMPACT_DISC_ID), CONSTRAINT FK_LABEL_ID FOREIGN KEY (LABEL_ID) REFERENCES CD_LABELS );
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