barcode generator in vb.net 2005 Figure 13-2: nonstraintsiquenessRelationship names in the CREATE TABLE statement in Software

Create Denso QR Bar Code in Software Figure 13-2: nonstraintsiquenessRelationship names in the CREATE TABLE statement

Figure 13-2: nonstraintsiquenessRelationship names in the CREATE TABLE statement
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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 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
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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 this chapter.)
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Uniqueness Constraints
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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. CREATE TABLE OFFICES (OFFICE INTEGER NOT NULL, CITY VARCHAR(15) NOT NULL, REGION VARCHAR(10) NOT NULL, MGR INTEGER, TARGET MONEY, SALES MONEY NOT NULL, PRIMARY KEY (OFFICE), CONSTRAINT HASMGR FOREIGN KEY (MGR) REFERENCES SALESREPS ON DELETE SET NULL, UNIQUE (CITY)) 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). CREATE TABLE OFFICES (OFFICE INTEGER NOT NULL PRIMARY KEY, CITY VARCHAR(15) NOT NULL UNIQUE, REGION VARCHAR(10) NOT NULL, MGR INTEGER REFERENCES SALESREPS, TARGET MONEY, SALES MONEY NOT NULL) Several of the major DBMS brands, including SQL Server, Informix, Sybase and DB2, support this shorthand.
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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 condition" (identical in form to a search condition in a SQL query) that is checked every
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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 OFFICES (OFFICE INTEGER NOT NULL, CITY VARCHAR(15) NOT NULL, REGION VARCHAR(10) NOT NULL, MGR INTEGER, TARGET MONEY, SALES MONEY NOT NULL, PRIMARY KEY (OFFICE), CONSTRAINT HASMGR FOREIGN KEY (MGR) REFERENCES SALESREPS ON DELETE SET NULL, CHECK (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 SALESREPS (EMPL_NUM INTEGER NOT NULL, NAME VARCHAR (15) NOT NULL, . . . CONSTRAINT WORKSIN FOREIGN KEY (REP_OFFICE) REFERENCES OFFICES ON DELETE SET NULL CONSTRAINT QUOTA_CAP CHECK ((HIRE_DATE < "01-JAN-88") OR (QUOTA <= 300000))) This check constraint capability is supported by many of the major DBMS brands.
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Physical Storage Definition *
The CREATE TABLE statement typically includes one or more optional clauses that specify physical storage characteristics for a table. Generally these clauses are used only by the database administrator to optimize the performance of a production database. By their nature these clauses are very specific to a particular DBMS. Although they are of little practical interest to most SQL users, the different physical storage structures provided by various DBMS products illustrate their different intended applications and levels of sophistication. Most of the personal computer databases provide very simple physical storage mechanisms. Many personal computer database products store an entire database within
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a single Windows file, or use a separate Windows file for each database table. They may also require that the entire table or database be stored on a single physical disk volume. Multi-user databases typically provide more sophisticated physical storage schemes to support improved database performance. For example, Ingres allows the database administrator to define multiple named locations, which are physical directories where database data can be stored. The locations can be spread across multiple disk volumes to take advantage of parallel disk input/output operations. You can optionally specify one or more locations for a table in the Ingres CREATE TABLE statement: CREATE TABLE OFFICES (table-definition) WITH LOCATION = (AREA1, AREA2, AREA3) By specifying multiple locations, you can spread a table's contents across several disk volumes for greater parallel access to the table. Sybase Adaptive Server offers a similar approach, allowing the database administrator to specify multiple named logical database devices that are used to store data. The correspondence between Sybase's logical devices and the actual physical disk drives of the computer system is handled by a Sybase utility program, and not within the SQL language. The Sybase CREATE DATABASE statement can then specify that a database should be stored on one or more database devices: CREATE DATABASE OPDATA ON DBFILE1, DBFILE2, DBFILE3 Within a given database device, Sybase then allows the database administrator to define logical segments, using one of the Sybase system-provided stored procedures. Finally, a Sybase CREATE TABLE statement can specify the segment where a table's data is to be stored: CREATE TABLE OFFICES (table-definition) ON SEGMENT SEG1A DB2 offers a similarly comprehensive scheme for managing physical storage, based on the concepts of tablespaces and nodegroups. A tablespace is a logical-level storage container, while nodegroups are defined more specifically in terms of physical storage. When you create a DB2 table, you can optionally assign it to a specific tablespace: CREATE TABLE OFFICES (table-definition) IN ADMINDB.OPSPACE Unlike Sybase, DB2 puts most of the management of these storage entities within the SQL language itself, through the CREATE TABLESPACE and CREATE NODEGROUP statements. A consequence is that these statements include operating system dependent specifications of filenames and directories, which vary from one supported DB2 operating system to another. Other clauses specify the DB2 buffer pool to be used, the overhead and transfer rate of the storage medium, and other characteristics closely related to the physical storage medium. DB2 uses this information in its performance optimization algorithms.
Removing a Table (DROP TABLE)
Over time the structure of a database grows and changes. New tables are created to represent new entities, and some old tables are no longer needed. You can remove an unneeded table from the database with the DROP TABLE statement, shown in Figure 133.
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