Oracle Schema Objects——Tables——Overview of Tables

Oracle Schema Objects

Overview of Tables

A table is the basic unit of data organization in an Oracle database.

表是Oracle数据库中的数据组织的基本单位。

A table describes an entity, which is something of significance about which information must be recorded.

一个表描述了一个实体，其相关重要信息必须被记录。

A table is either permanent or temporary.

表要么是永久的，要么是临时的。

Relational tables

关系表

Relational tables have simple columns and are the most common table type.

关系表具有简单的列，是最常见的表类型。

可以创建如下关系表：

堆组织表

heap-organized table

A heap-organized table does not store rows in any particular order. The CREATE TABLE statement creates a heap-organized table by default.

它不会以任何特定顺序存储行

索引组织表

index-organized table

An index-organized table orders rows according to the primary key values. For some applications, index-organized tables enhance performance and use disk space more efficiently.

它按主键值对行进行排序。对于某些应用程序，索引组织表可以增强性能并更有效地使用的磁盘空间

外部表

external table

An external table is a read-only table whose metadata is stored in the database but whose data in stored outside the database.是一个只读表，它的元数据存储在数据库中，但其数据存储在数据库外

Object tables

对象表

The columns correspond to the top-level attributes of an object type.

列对应于对象类型的顶层属性。

Columns and Rows

行和列

A table definition includes a table name and set of columns.

表的定义包括表名称和列集。

1、A column identifies an attribute of the entity described by the table.

列标识由表所描述的实体的一个属性。

virtual column

虚拟列

A table can contain a virtual column, which unlike a nonvirtual column does not consume disk space.

表可以包含虚拟的列，与非虚拟列不一样，虚拟列不占用磁盘空间。

The database derives the values in a virtual column on demand by computing a set of user-specified expressions or functions.

数据库通过计算一组用户指定的表达式或函数，按需派生出虚拟的列值。

For example, the virtual column income could be a function of the salary and commission_pct columns.

例如虚拟列 income 可能是 salary 列和 commission_pct 列的一个函数。

2、 A row is a collection of column information corresponding to a record in a table.

行是对应于表中某条记录的列信息的集合。

create a table

创建表

1、In general, you give each column a column name, a data type, and a width when you create a table.

一般地，当创建一个表时，给每列一个列名称、数据类型、宽度。

2、CREATE TABLE Statements

CREATE TABLE employees

    ( employee_id    NUMBER(6)

    , first_name     VARCHAR2(20)

    , last_name      VARCHAR2(25)

         CONSTRAINT     emp_last_name_nn NOT NULL

    , email          VARCHAR2(25)

        CONSTRAINT     emp_email_nn NOT NULL

    , phone_number   VARCHAR2(20)

    , hire_date      DATE

        CONSTRAINT     emp_hire_date_nn NOT NULL

    , job_id         VARCHAR2(10)

        CONSTRAINT     emp_job_nn NOT NULL

    , salary         NUMBER(8,2)

    , commission_pct NUMBER(2,2)

    , manager_id     NUMBER(6)

    , department_id NUMBER(4)

    , CONSTRAINT     emp_salary_min

                     CHECK (salary > 0)

    , CONSTRAINT     emp_email_uk

                     UNIQUE (email)

    ) ;

3、ALTER TABLE Statements

ALTER TABLE employees

ADD ( CONSTRAINT     emp_emp_id_pk

                       PRIMARY KEY (employee_id)

    , CONSTRAINT     emp_dept_fk

                       FOREIGN KEY (department_id)

                         REFERENCES departments

    , CONSTRAINT     emp_job_fk

                       FOREIGN KEY (job_id)

                         REFERENCES jobs (job_id)

    , CONSTRAINT     emp_manager_fk

                       FOREIGN KEY (manager_id)

                         REFERENCES employees

    ) ;

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Oracle Schema Objects——Tables——Overview of Tables

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