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OP database
FIGURE 13-11
A multilocation architecture
If the database is in the user s current directory (which is often the case), the statement simplifies to:
DECLARE DATABASE FILENAME 'OP';
PART IV
Some of the DBMS brands that use this scheme allow you to have access to several databases concurrently, even if they don t support queries across database boundaries. Again, the most common technique used to distinguish among the multiple databases is to use a superqualified table name. Since two databases in two different directories can have the same name, it s also necessary to introduce a database alias to eliminate ambiguity. These VAX SQL statements open two different Rdb/VMS databases that happen to have the same name:
DECLARE DATABASE FILENAME DECLARE DATABASE FILENAME OP1 'SYS$ROOT:[PRODUCTION\]OP' OP2 'SYS$ROOT:[DEVELOPMENT.TEST]OP';
The statements assign the aliases OP1 and OP2 to the two databases, and these aliases are used to qualify table names in subsequent VAX SQL statements. As this discussion shows, there can be a tremendous variety in the way various DBMS brands organize their databases and provide access to them. This area of SQL is one of the most nonstandard, and yet it is often the first one that a user encounters when trying to access a database for the first time. The inconsistencies also make it impossible to transparently move programs developed for one DBMS to another, although the conversion process is usually tedious rather than complex.
Part IV:
Database Structure
Databases on Multiple Servers
With the rise of database servers and local area networks, the notion of database location embodied in the multilocation architecture is naturally extended to the notion of a physical database server. In practice, most DBMS products today appear to be converging on a multidatabase architecture implemented within a physical server. At the highest level, a database is associated with a named server on the network. Within the server, there can be multiple named databases. The mapping of server names to physical server locations is handled by the networking software. The mapping of database names to physical files or file systems on a server is handled by the DBMS software.
Database Structure and the ANSI/ISO Standard
The ANSI/ISO SQL1 standard made a very strong distinction between the SQL Data Manipulation Language and Data Definition Language, defining them effectively as two separate languages. The standard did not require that the DDL statements be accepted by the DBMS during its normal operation. One of the advantages of this separation of the DML and DDL was that the standard permitted a static database structure like that used by older hierarchical and network DBMS products, as shown in Figure 13-12. The database structure specified by the SQL1 standard was fairly straightforward. Collections of tables were defined in a database schema, associated with a specific user. The simple database shown in Figure 13-12 has two schemas. One schema is associated with (the common terminology is owned by) a user named Joe, and the other is owned by Mary. Joe s schema contains two tables, named PEOPLE and PLACES. Mary s schema also contains two tables, named THINGS and PLACES. Although the database contains two tables named PLACES, it s possible to tell them apart because they have different owners. Starting with SQL2, the SQL standard significantly extends the notion of database definition and database schemas. As previously noted, the SQL standard now requires that data definition statements be executable by an interactive SQL user or by a SQL program. With this capability, changes to the database structure can be made at any time, not just when the database is created. In addition, the SQL1 concepts of schemas and users (officially called authorization-ids in the standard) has been significantly expanded. Figure 13-13 shows the high-level database structure specified by current versions of the SQL standard. The highest-level database structure described by the SQL standard is the SQL-environment. This is a conceptual collection of the database entities associated with a DBMS implementation that conforms to the SQL standard. The standard doesn t specify how a SQL-environment is created; that depends on the particular DBMS implementation. The standard defines these components of a SQL-environment: DBMS software that conforms to the SQL standard. Named users (called authorization-ids in the standard) who have the privileges to perform specific actions on the data and structures within the database. Program modules that are used to access the database. The SQL standard specifies the actual execution of SQL statements in terms of a module language, which in practice is not used by most major commercial SQL products. No matter how the SQL programs are actually created, however, the standard says that, conceptually, the SQL-environment includes the program s database access code.
13:
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