原语Oracle数据库中段的英文原语解析（oracle中段的英文）

Understanding Segment-related SQL Commands in Oracle Database

Oracle Database is a powerful and widely used relational database management system. It offers a wide range of SQL commands to manage and manipulate various database objects such as tables, indexes, views, and segments. In this article, we’ll focus on understanding the SQL commands related to database segments in Oracle.

A segment in Oracle Database is a logical storage structure that corresponds to a physical file on the hard disk. Segments are used to store and manage data within a tablespace, which is a logical grouping of segments. There are several types of segments in Oracle Database, including table segments, index segments, lob segments, temp segments, and undo segments. Let’s take a look at some of the SQL commands used to manage these segments.

1. CREATE TABLE

The CREATE TABLE command is used to create a new table segment in Oracle Database. Here’s an example:

CREATE TABLE employees(
   emp_id NUMBER(4),
   name VARCHAR2(20),
   salary NUMBER(7,2)
);

This command creates a new table segment named “employees” with three columns: emp_id, name, and salary.

2. ALTER TABLE

The ALTER TABLE command is used to modify an existing table segment in Oracle Database. Here are some examples:

ALTER TABLE employees ADD hire_date DATE;
ALTER TABLE employees MODIFY salary NUMBER(8,2);
ALTER TABLE employees DROP COLUMN name;

These commands add a new hire_date column to the employees table, modify the salary column to allow for more digits, and delete the name column from the table.

3. CREATE INDEX

The CREATE INDEX command is used to create a new index segment in Oracle Database. Here’s an example:

CREATE INDEX emp_id_idx ON employees(emp_id);

This command creates a new index segment called “emp_id_idx” on the emp_id column of the employees table.

4. ALTER INDEX

The ALTER INDEX command is used to modify an existing index segment in Oracle Database. Here’s an example:

ALTER INDEX emp_id_idx REBUILD;

This command rebuilds the emp_id_idx index segment, which can improve its performance.

5. CREATE TABLESPACE

The CREATE TABLESPACE command is used to create a new tablespace in Oracle Database. Here’s an example:

CREATE TABLESPACE data_ts
   DATAFILE '/u01/app/oracle/data_ts.dbf'
   SIZE 100M;

This command creates a new tablespace called “data_ts” with a 100MB data file located at the specified path.

6. ALTER TABLESPACE

The ALTER TABLESPACE command is used to modify an existing tablespace in Oracle Database. Here are some examples:

ALTER TABLESPACE data_ts ADD DATAFILE '/u01/app/oracle/data_ts2.dbf' SIZE 100M;
ALTER TABLESPACE data_ts RESIZE DATAFILE '/u01/app/oracle/data_ts.dbf' SIZE 200M;

These commands add a new data file to the data_ts tablespace and resize the existing data file to accommodate more data.

In conclusion, managing database segments is an important aspect of Oracle Database administration. With the SQL commands discussed in this article, you can create, modify, and manage various types of segments in your database. By mastering these commands, you can optimize the performance and efficiency of your database.