Oracle DBA宝典：创建表与索引详解
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"Oracle DBA宝典第13章深入解析数据库管理与表设计"
在Oracle DBA的日常工作中，表格管理是核心环节，本章详细介绍了如何运用SQL命令来构建和优化数据库中的数据结构。首先，创建表和索引是基础，包括使用CREATE TABLE语句定义列（存储数据类型），设置主键和外键约束以确保数据完整性，以及定义其他业务规则所必需的约束条件。虽然现代DBA可能更倾向于使用图形用户界面（GUI）工具，如Enterprise Manager的Schema Manager，但章节中也包含了命令行操作的教程，以便于理解和实践。
SQL命令在此章节中扮演着关键角色，尽管复杂，但提供了丰富的实例以便理解。不过，为了完整掌握语法细节，所有命令都收录在附录A“SQL语句参考”中，鼓励读者查阅以深化理解。
理解表格结构至关重要，因为它是关系型数据库的基石。一个典型的表格包含以下要素：
1. 列（Columns）：定义了存储在表中的具体数据类型，比如整数、字符串或日期，这些列是数据的基本单元。
2. 约束（Constraints）：通过限制可存储的数据范围，确保数据质量，如NOT NULL约束确保字段不能为空，UNIQUE约束防止重复值，而PRIMARY KEY约束则用于标识唯一的记录。
3. 索引（Indexes）：是提高查询性能的关键，它们允许快速访问表中基于特定列值的数据，对主键和唯一键尤其重要，因为Oracle会自动为这些键创建索引。
Oracle超越了传统的关系型数据库，作为对象关系型数据库系统，它支持更复杂的对象管理，如视图、存储过程和触发器等。这些高级特性使得数据管理更加灵活且高效。通过学习本章内容，DBA不仅能够掌握基础的表设计，还能为后续的数据库优化和性能调优打下坚实的基础。熟练掌握这些技能对于维护大型Oracle数据库环境至关重要。
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Part III ✦ Nuts and Bolts
Constraints
Constraints are rules about your data that you define at the database level. Constraints
are declarative. You don’t specify the process for enforcing a rule; you simply specify
what that rule is to be. Oracle supports these types of constraints:
✦ Primary key
✦ Unique key
✦ Foreign key
✦ Check
Primary key constraints identify the set of columns whose values uniquely identify
a record in a table. Oracle won’t allow two records to have the same primary key
value. For example, the
ID_NO column is the primary key of the AQUATIC_ANIMAL
table. Because ID_NO has been defined as the primary key, Oracle will not allow two
records to have the same value in that column. Furthermore, primary key columns
become required columns—they can never be null.
Unique key constraints identify sets of columns that must be unique for each row
in a table. Unique key constraints are similar to primary key constraints, and they
often represent alternative primary key choices. The one difference between a
unique key and a primary key is that columns in a unique key may be null.
When you create a primary key or unique key constraint on a table, Oracle will cre-
ate a unique index to enforce that constraint. The name of the index will match
the name that you give the constraint.
Foreign key constraints are used to link two tables that contain related information.
They are most often used in parent-child relationships, such as the one between
the
TANK table and the AQUATIC_ANIMAL table. A foreign key constraint can be
placed on the
AQUATIC_ANIMAL table, requiring that any TANK_NO value match
a record in the
TANK table. In this way, you can prevent erroneous tank numbers
from being entered in the
AQUATIC_ANIMAL table. Foreign keys must be linked to
a corresponding primary or unique key.
Check constraints allow you to define an arbitrary condition that must be true before
a row can be saved in a table. The most common check constraint is the
NOT NULL
constraint, which requires that a column have a value. You can supply almost any
arbitrary expression for a check constraint, provided that it returns a value of
TRUE
or FALSE.
Note
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