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Creating a Database
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DATABASE STRUCTURE
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Figure 13-2.
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Relationship names in the CREATE TABLE statement
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update rules, and the actions that can trigger them, are described in 11. The DBMS enforces the default rules (NO ACTION) if no rule is explicitly specified. If you want to create two or more tables from a referential cycle (like the OFFICES and SALESREPS tables in the sample database), you cannot include the foreign key definition in the first CREATE TABLE statement because the referenced table does not yet exist. The DBMS will reject the attempted CREATE TABLE statement with an error saying that the table definition refers to an undefined table. Instead, you must create the first table without its foreign key definition and add the foreign key later using the ALTER TABLE statement. (The SQL2 standard and several of the major DBMS products offer a different solution to this problem with the CREATE SCHEMA statement, which creates an entire set of tables at once. This statement and the other database objects that are included within a SQL2 schema are described later in the SQL2 Schemas section.)
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The SQL2 standard specifies that uniqueness constraints are also defined in the CREATE TABLE statement, using the UNIQUE clause shown in Figure 13-1. Here is a CREATE TABLE statement for the OFFICES table, modified to require unique CITY values: Define the OFFICES table with a uniqueness constraint.
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CREATE TABLE (OFFICE CITY REGION MGR TARGET SALES PRIMARY KEY CONSTRAINT FOREIGN KEY REFERENCES ON DELETE UNIQUE OFFICES INTEGER NOT NULL, VARCHAR(15) NOT NULL, VARCHAR(10) NOT NULL, INTEGER, MONEY, MONEY NOT NULL, (OFFICE), HASMGR (MGR) SALESREPS SET NULL, (CITY))
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If a primary key, foreign key, uniqueness constraint, or check constraint involves a single column, the ANSI/ISO standard permits a shorthand form of the definition. The primary key, foreign key, uniqueness constraint, or check constraint is simply added to the end of the column definition, as shown in this example: Define the OFFICES table with a uniqueness constraint (ANSI/ISO syntax).
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CREATE TABLE (OFFICE CITY REGION MGR INTEGER TARGET SALES OFFICES INTEGER NOT NULL PRIMARY KEY, VARCHAR(15) NOT NULL UNIQUE, VARCHAR(10) NOT NULL, REFERENCES SALESREPS, MONEY, MONEY NOT NULL)
Several of the major DBMS brands, including SQL Server, Informix, Sybase, and DB2, support this shorthand.
Check Constraints
Another SQL2 data integrity feature, the check constraint (described in 11) is also specified in the CREATE TABLE statement. A check constraint specifies a check
13:
Creating a Database
condition (identical in form to a search condition in a SQL query) that is checked every time an attempt is made to modify the contents of the table (with an INSERT, UPDATE, or DELETE statement). If the check condition remains TRUE after the modification, it is allowed; otherwise, the DBMS disallows the attempt to modify the data and returns an error. The following is a CREATE TABLE statement for the OFFICES table, with a very simple check condition to make sure the TARGET for the office is greater than $0.00. Define the OFFICES table with a uniqueness constraint.
CREATE TABLE (OFFICE CITY REGION MGR TARGET SALES PRIMARY KEY CONSTRAINT FOREIGN KEY REFERENCES ON DELETE CHECK OFFICES INTEGER NOT NULL, VARCHAR(15) NOT NULL, VARCHAR(10) NOT NULL, INTEGER, MONEY, MONEY NOT NULL, (OFFICE), HASMGR (MGR) SALESREPS SET NULL, (TARGET >= 0.00))
You can optionally specify a name for the check constraint, which will be used by the DBMS when it reports an error if the constraint is violated. Here is a slightly more complex check constraint for the SALESREPS table to enforce the rule Salespeople whose hire date is later than January 1, 1988, shall not be assigned quotas higher than $300,000. The CREATE TABLE statement names this constraint QUOTA_CAP:
CREATE TABLE (EMPL_NUM NAME . . . CONSTRAINT FOREIGN KEY REFERENCES ON DELETE CONSTRAINT SALESREPS INTEGER NOT NULL, VARCHAR (15) NOT NULL,
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