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The definition of each column within a table tells the DBMS whether the data for the column is allowed to be missing that is, whether the column is allowed to have a NULL value. In most of the major DBMS brands and in the SQL standard, the default is to allow missing data for a column. If the column must contain a legal data value for every row of a table, then its definition must include the NOT NULL clause. The Sybase DBMS products and Microsoft SQL Server use the opposite convention, assuming that NULL values are not allowed unless the column is explicitly declared NULL or the default nullability mode defined for the database is set to allow NULLs by default. The SQL2 standard and many of the major SQL DBMS products support default values for columns. If a column has a default value, it is specified within the column definition. For example, here is a CREATE TABLE statement for the OFFICES table that specifies default values:
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Define the OFFICES table with default values (ANSI/ISO syntax).
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CREATE TABLE (OFFICE CITY REGION MGR TARGET SALES OFFICES INTEGER NOT NULL, VARCHAR(15) NOT NULL, VARCHAR(10) NOT NULL DEFAULT 'Eastern', INTEGER DEFAULT 106, MONEY DEFAULT NULL, MONEY NOT NULL DEFAULT 0.00)
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With this table definition, only the office number and the city need to be specified when you insert a new office. The region defaults to Eastern, the office manager to Sam Clark (employee number 106), the sales to zero, and the target to NULL. Note that the target would default to NULL even without the DEFAULT NULL specification.
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In addition to defining the columns of a table, the CREATE TABLE statement identifies the table s primary key and the table s relationships to other tables in the database. The PRIMARY KEY and FOREIGN KEY clauses handle these functions. These clauses have been supported by the IBM SQL databases for some time and have been added to the ANSI/ISO specification. Most major SQL products support them. The PRIMARY KEY clause specifies the column or columns that form the primary key for the table. Recall from 4 that this column (or column combination) serves as a unique identifier for each row of the table. The DBMS automatically requires that the primary key value be unique in every row of the table. In addition, the column definition for every column in the primary key must specify that the column is NOT NULL. The FOREIGN KEY clause specifies a foreign key in the table and the relationship that it creates to another (parent) table in the database. The clause specifies: I The column or columns that form the foreign key, all of which are columns of the table being created. I The table that is referenced by the foreign key. This is the parent table in the relationship; the table being defined is the child. I An optional name for the relationship. The name is not used in any SQL data manipulation statements, but it may appear in error messages and is required if you want to be able to drop the foreign key later. I How the DBMS should treat a NULL value in one or more columns of the foreign key, when matching it against rows of the parent table. I An optional delete rule for the relationship (CASCADE, SET NULL, SET DEFAULT, or NO ACTION as described in 11), which determines the action to take when a parent row is deleted.
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I An optional update rule for the relationship as described in 11, which determines the action to take when part of the primary key in a parent row is updated. I An optional check constraint, which restricts the data in the table so that its rows meet a specified search condition. 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: Define the ORDERS table with its primary and foreign keys.
CREATE TABLE (ORDER_NUM ORDER_DATE CUST REP MFR PRODUCT QTY AMOUNT PRIMARY KEY CONSTRAINT FOREIGN KEY REFERENCES ON DELETE CONSTRAINT FOREIGN KEY REFERENCES ON DELETE CONSTRAINT FOREIGN KEY REFERENCES ON DELETE ORDERS INTEGER NOT NULL, DATE NOT NULL, INTEGER NOT NULL, INTEGER, CHAR(3) NOT NULL, CHAR(5) NOT NULL, INTEGER NOT NULL, MONEY NOT NULL, (ORDER_NUM), PLACEDBY (CUST) CUSTOMERS CASCADE, TAKENBY (REP) SALESREPS SET NULL, ISFOR (MFR, PRODUCT) PRODUCTS RESTRICT)
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 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
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