Creating a Database in Software

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Creating a Database
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In a large mainframe or enterprise-level network DBMS installation, the corporate database administrator is solely responsible for creating new databases. On smaller workgroup DBMS installations, individual users may be allowed to create their own personal databases, but it's much more common for databases to be created centrally and then accessed by individual users. If you are using a personal computer DBMS, you are probably both the database administrator and the user, and you will have to create the database(s) that you use personally.
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The SQL1 standard specified the SQL language used to describe a database structure, but it did not specify how databases are created, because each DBMS brand had taken a slightly different approach. Those differences persist in present-day mainstream DBMS products. The techniques used by these SQL products illustrate the differences: IBM's DB2 has a simple default database structure. A DB2 database is associated with a running copy of the DB2 server software, and users access the database by connecting to the DB2 server. A DB2 "database" is thus effectively defined by an installation of the DB2 software on a particular computer system. Oracle by default creates a database as part of the Oracle software installation process, like DB2. For the most part, user tables are always placed in this single, system-wide database, which is named by an Oracle configuration file and associated with this particular copy of the Oracle server software. More recent versions of Oracle have been extended with a CREATE DATABASE statement for defining database names. Ingres includes a special utility program, called CREATEDB, which creates a new Ingres database. A companion program, DESTROYDB, erases an unneeded database. Microsoft SQL Server and Sybase Adaptive Server include a CREATE DATABASE statement as part of their data definition language. A companion DROP DATABASE statement destroys previously created databases. These statements can be used with interactive or programmatic SQL. The names of these databases are tracked in a special "master" database that is associated with a single installation of SQL Server. Database names must be unique within this SQL Server installation. Options to the CREATE DATABASE statement specify the physical I/O device on which the database is to be located. Informix Universal Server supports CREATE DATABASE and DROP DATABASE SQL statements as well. An option to the CREATE DATABASE statement allows the database to be created in a specific dbspace, which is a named area of disk storage controlled by the Informix software. Another option controls the type of database logging to be performed for the new database, with trade-offs between performance and data integrity during system failures. The SQL2 standard specifically avoids a specification of the term "database" because it is so overloaded with contradictory meanings from DBMS products. SQL2 uses the term catalog to describe a named collection of tables that is called a "database" by most popular DBMS brands. (Additional information about the database structure specified by the SQL2 standard is provided later in this chapter.) The standard does not specify how a catalog is created or destroyed, and specifically says that creation or destruction is implementationdependent. It also indicates how many catalogs there are, and whether individual SQL statements that can access data from different catalogs are implementation-defined. In practice, as shown by the preceding examples, many of the major DBMS vendors have moved toward the use of a CREATE DATABASE/DROP DATABASE statement pair.
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The most important structure in a relational database is the table. In a multi-user production database, the major tables are typically created once by the database administrator and then used day after day. As you use the database you will often find it convenient to define your own tables to store personal data or data extracted from other tables. These tables may be temporary, lasting only for a single interactive SQL session, or more permanent, lasting weeks or months. In a personal computer database, the table structure is even more fluid. Because you are both the user and the database administrator, you can create and destroy tables to suit your own needs, without worrying about other users.
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Creating a Table (CREATE TABLE)
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The CREATE TABLE statement, shown in Figure 13-1, defines a new table in the database and prepares it to accept data. The various clauses of the statement specify the elements of the table definition. The syntax diagram for the statement appears complex because there are so many parts of the definition to be specified and so many options for each element. In addition, some of the options are available in some DBMS brands or in the SQL2 standard, but not in other brands. In practice, creating a new table is relatively straightforward.
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