create barcode using vb.net The DROP TABLE Command in Java

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7.10 The DROP TABLE Command
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You can drop your tables with the DROP TABLE command. Figure 7-18 shows the syntax diagram for the DROP TABLE command.
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Figure 7-18. A DROP TABLE command syntax diagram
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DATA DEFINITION, PART II
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Unless you have specific system privileges, you cannot drop tables owned by other database users. Also, you cannot roll back a DROP TABLE command. As you ve learned in previous chapters, this is true for all DDL statements (CREATE, ALTER, and DROP). Errare humanum est, as the Romans said. Because human errors occur occasionally, Oracle Database 10g introduced the concept of the database recycle bin. By default, all dropped tables (and their dependent objects) initially end up in the recycle bin. You can query the recycle bin using the [USER_]RECYCLEBIN view, as shown in Listing 7-18. To make sure we start with an empty recycle bin, we begin the experiment with a PURGE command. Listing 7-18. Dropping Tables and Querying the Recycle Bin SQL> purge recyclebin; Recyclebin purged. SQL> drop table history; Table dropped. SQL> select object_name, original_name, droptime 2 from recyclebin; OBJECT_NAME -----------------------------BIN$mlRH1je9TBOeVEUhukIpCw==$0 BIN$EETkZCY0RSKCR3BhtF9cJw==$0 SQL> As you can see, the objects are renamed, but the original names are kept as well. There is one entry for the HISTORY table and one entry for the primary key index. You can recover tables (and optionally rename them) from the recycle bin by using the FLASHBACK TABLEcommand: SQL> flashback table history to before drop 2 [rename to <new name>]; Flashback complete. SQL> ORIGINAL_NAME ---------------------H_PK HISTORY DROPTIME ------------------2004-07-01:20:22:23 2004-07-01:20:22:23
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Caution There is no guarantee the FLASHBACK TABLE command always succeeds. The recycle bin can be purged explicitly (by a database administrator) or implicitly (by the Oracle DBMS).
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If you want to drop a table and bypass the recycle bin, you can use the PURGE option of the DROP TABLE command, as shown in Figure 7-18. If you drop a table, you implicitly drop certain dependent database objects, such as indexes, triggers, and table privileges granted to other database users. You also invalidate certain other database objects, such as views and packages. Keep this in mind during database reorganizations. To re-create a
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table, it is not enough to simply issue a CREATE TABLE command after a DROP TABLE command. You need to reestablish the full environment around the dropped table. If you issue a DROP TABLE command, you may get the following error message if other tables contain foreign key constraints referencing the table that you are trying to drop: ORA-02449: unique/primary keys in table referenced by foreign keys Try to drop the EMPLOYEES table, and see what happens. You can solve this problem by using the CASCADE CONSTRAINTS option, as shown in Figure 7-18. Note, however, that this means that all offending foreign key constraints are dropped, too.
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7.11 The TRUNCATE Command
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The TRUNCATE command allows you to delete all rows from a table. Figure 7-19 shows the syntax diagram for the TRUNCATE command.
Figure 7-19. A TRUNCATE command syntax diagram The default behavior is DROP STORAGE, as indicated by the underlining in Figure 7-19. Compared with DROP TABLE (followed by a CREATE TABLE), the big advantage of TRUNCATE is that all related indexes and privileges survive the TRUNCATE operation. This command has two possible advantages over the DELETE command: the performance (response time) is typically better for large tables, and you can optionally reclaim the allocated space. However, there is a price to pay for these two advantages: you cannot perform a ROLLBACK to undo a TRUNCATE, because TRUNCATE is a DDL command. The Oracle DBMS treats DDL commands as single-statement transactions and commits them immediately.
7.12 The COMMENT Command
The COMMENT command allows you to add clarifying (semantic) explanations about tables and table columns to the data dictionary. Figure 7-20 shows the syntax diagram for this command.
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