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SELECT NAME, QUOTA, SALES FROM SAMPLE@CENTRALHOST:SALEREPS WHERE SALES > QUOTA
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The database name appears at the beginning of the table name (as an additional qualifier before the colon). If the database is remote, then the server name appears following the at sign (@) after the database name.
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With any of the remote database-naming conventions that extend the usual SQL table and view names, the additional qualifiers can quickly become annoying or confusing. For example, if two tables in the remote database have columns with the same names, any query involving both tables must use qualified column names and the table name qualifiers now have the remote database qualification as well. Here s a qualified Informix column name for the NAME column in the remote SALESREPS table owned by the user JOE in a remote database named SAMPLE on the remote Informix server CENTRALHOST:
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SAMPLE@CENTRALHOST.JOE.SALESREPS.NAME
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A single column reference has grown to half a line of SQL text! For this reason, table aliases are frequently used in SQL statements involving remote database access. Synonyms and aliases (described in 16) are also very useful for providing more transparent access to remote databases. Here s an Informix synonym definition that could be established by a user or a database administrator:
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CREATE SYNONYM REMOTE_REPS FOR SAMPLE@CENTRALHOST.JOE.SALESREPS
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The equivalent Oracle synonym definition is:
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CREATE SYNONYM REMOTE_REPS FOR JOE.SALESREPS@CENTRALHOST
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With this synonym in place, the preceding qualified column name becomes simply:
REMOTE_REPS.NAME
Any query referencing the REMOTE_REPS table and its columns is actually a remote database query, but that fact is transparent to the user. In practice, most database installations with frequently accessed remote tables will have a set of synonyms defined for them. Most of the DBMS brands support both public synonyms (available to all users) and private synonyms that are created for a specific user or group of users. With this structure, synonyms can become an additional part of the remote access security mechanism, limited to only those users with a real need for remote access. Several DBMS brands take the synonym capability for transparent database access one step further and permit views in the local database that are defined in terms of remote database tables. Here is an Oracle view definition that creates a view called
23:
SQL Networking and Distributed Databases
EAST_REPS in the local database. The view is a subset of information from the remote sample database: Create a local view defined in terms of two remote tables.
CREATE VIEW SELECT FROM WHERE AND EAST_REPS AS EMPL_NUM, NAME, AGE, CITY SALESREPS@CENTRALHOST, OFFICES@CENTRALHOST REP_OFFICE = OFFICE REP_OFFICE BETWEEN 11 AND 19
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After this view has been defined, a user can pose queries in terms of the EAST_REPS view, without worrying about database links or remote table names. The view not only provides transparent remote access, but also hides from the user the remote join operation between the OFFICES and SALESREPS tables. Transparent access to remote data, provided by views and synonyms, is usually considered a very desirable characteristic. It does have one drawback, however. Because the remote aspect of the database access is now hidden, the network overhead created by the access is also hidden. Therefore, the possibility of a user or programmer inadvertently creating a great deal of network traffic through very large queries is increased. The database administrator must make this trade-off when deciding whether to permit remote transparent synonyms and views. Transparent remote access also inevitably raises one additional question: since the remote tables now appear as if they are local, can the user pose queries that involve both remote and local tables That is, can a join cross the database server boundaries and relate information from the remote database and the local database Even more serious questions are posed when the SQL transaction scheme is considered. If a database permits transparent access to a remote database, then is a user allowed to update a row in the local database and insert a row in the remote database, and then decide to roll back the transaction Since the remote resources have been made to appear as if they are local, it seems that the answer should be: Of course the local and remote databases together should appear as if they were just one local, integrated database. In fact, supporting such distributed queries and transactions adds a major new level of complexity (and potentially huge network data transmission overhead) to the remote access. Because of this, although several commercial DBMS systems support distributed queries and transactions, they are not heavily used in practice. These capabilities, and their overhead implications, are more fully discussed later, in the Distributed Database Access section. The next section discusses a practical alternative duplicating data, or database replication that is much more frequently used in practice.
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