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Creating a Database
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Here is an expanded CREATE TABLE statement for the ORDERS table, which includes a definition of its primary key and the three foreign keys that it contains: De ne the ORDERS table with its primary and foreign keys.
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CREATE TABLE ORDERS (ORDER_NUM INTEGER NOT NULL, ORDER_DATE DATE NOT NULL, CUST INTEGER NOT NULL, REP INTEGER, MFR CHAR(3) NOT NULL, PRODUCT CHAR(5) NOT NULL, QTY INTEGER NOT NULL, AMOUNT DECIMAL(9,2) NOT NULL, PRIMARY KEY (ORDER_NUM), CONSTRAINT PLACEDBY FOREIGN KEY (CUST) REFERENCES CUSTOMERS ON DELETE CASCADE, CONSTRAINT TAKENBY FOREIGN KEY (REP) REFERENCES SALESREPS ON DELETE SET NULL, CONSTRAINT ISFOR FOREIGN KEY (MFR, PRODUCT) REFERENCES PRODUCTS ON DELETE RESTRICT);
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Figure 13-2 shows the three relationships created by this statement and the names it assigns to them. In general, it s a good idea to assign a relationship name, because it helps to clarify the relationship created by the foreign key. For example, each order was placed by the customer whose number appears in the CUST column of the ORDERS table. The relationship created by this column has been given the name PLACEDBY. When the DBMS processes the CREATE TABLE statement, it checks each foreign key definition against the definition of the table that it references. The DBMS makes sure that the foreign key and the primary key of the referenced table agree in the number of columns they contain and in their data types. The referenced table must already be defined in the database for this checking to succeed. Note that the FOREIGN KEY clause also specifies the delete and update rules that are to be enforced for the parent/child table relationship that it creates. Delete and 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 by using the ALTER TABLE statement. (The SQL 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 included within a SQL schema are described later in the Schemas section.)
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Relationship names in the CREATE TABLE statement
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Uniqueness Constraints
The SQL 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: De ne the OFFICES table with a uniqueness constraint.
CREATE TABLE OFFICES (OFFICE INTEGER NOT CITY VARCHAR(15) NOT REGION VARCHAR(10) NOT MGR INTEGER, TARGET DECIMAL(9,2), SALES DECIMAL(9,2) NOT PRIMARY KEY (OFFICE), CONSTRAINT HASMGR FOREIGN KEY (MGR) REFERENCES SALESREPS ON DELETE SET NULL, UNIQUE (CITY)); NULL, NULL, NULL, NULL,
13:
Creating a Database
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: De ne the OFFICES table with a uniqueness constraint (ANSI/ISO syntax).
CREATE TABLE (OFFICE CITY REGION MGR TARGET SALES OFFICES INTEGER NOT NULL PRIMARY KEY, VARCHAR(15) NOT NULL UNIQUE, VARCHAR(10) NOT NULL, INTEGER REFERENCES SALESREPS, DECIMAL(9,2), DECIMAL(9,2) NOT NULL);
Several of the major DBMS brands, including SQL Server, Informix, Sybase, and DB2, support this shorthand.
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