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Renaming a Table Column
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You can easily rename table columns using the RENAME COLUMN command. For example, the following command will rename the retired column in the emp table to non_active. Note that you can also rename the column constraints, if you wish. SQL> ALTER TABLE emp RENAME COLUMN retired TO non_active; Table altered. SQL>
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You can rename tempfiles as well as data files and the redo log file, using the ALTER DATABASE command.
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On occasion, an application developer may want to rename a table. Renaming a table is straightforward: SQL> ALTER TABLE emp RENAME TO emp; Table altered. SQL>
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Removing All the Data from a Table
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To remove all the rows from a table, you can use the TRUNCATE command, which, contrary to its name, doesn t abbreviate or shorten anything it summarily removes all the rows very quickly. TRUNCATE is a DDL command, so it can t be undone by using the ROLLBACK command. You can also remove all the rows in a table with the DELETE * FROM TABLE command, and because this is a DML command, you can roll back the deletion if you desire. However, because the DELETE command writes all changes to the undo segments, it takes a much longer time to execute. Because the TRUNCATE command doesn t have to bother with the undo segments, it executes in a few seconds, even for the largest tables. Here s an example of the TRUNCATE command in action: SQL> SELECT COUNT(*) FROM test; COUNT(*) ----------31
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CHAPTER 5 SCHEMA MANAGEMENT
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SQL> TRUNCATE TABLE test; Table truncated. SQL> SELECT COUNT(*) FROM test; COUNT(*) -----------0 SQL>
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Creating a New Table with the CTAS Option
To create a new table that is identical to an existing table, or to create a new table that includes only some rows and columns from another table, you can use the CREATE TABLE AS SELECT * FROM (CTAS) command. With this command, you can load a portion of an existing table into a new table by using WHERE conditions, or you can load all the data of the old table into the newly created table by simply using SELECT * FROM clause, as shown in the following code snippet: SQL> CREATE TABLE emp_new AS SELECT * FROM emp; Table created. SQL> If the table has millions of rows, and your time is too limited to use the simple CTAS method, there are a couple of ways to speed up the creation of new tables that contain large amounts of data. If the table you re creating is empty, you don t need to be concerned with the speed with which it s created it s created immediately. But if you re loading the new table from an existing table, you can benefit from using the PARALLEL and NOLOGGING options, which speed up the loading of large tables. The PARALLEL option enables you to do your data loading in parallel by several processes, and the NOLOGGING option instructs Oracle not to bother logging the changes to the redo log files and rollback segments (except the very minimum necessary for housekeeping purposes). Here s an example: SQL> CREATE TABLE employee_new 2 AS SELECT * FROM employees 3 PARALLEL DEGREE 4 4*NOLOGGING; Table created. SQL> The other method you can use to save time during table creation is to simply move a table from one tablespace to another. You can take advantage of the moving operation to change any storage parameters you wish. Here s an example of the ALTER TABLE . . . MOVE command, which enables you to move tables between tablespaces rapidly. In this example, the employee table is moved from its present tablespace to a new tablespace: SQL> ALTER TABLE employee MOVE new_tablespace; When you move a table, the ROWIDs of the rows change, thus making the indexes on the table unusable. You must either re-create the indexes or rebuild them after you move the table.
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