• Introduction to Data Integrity
• Overview of Integrity Constraints
• Types of Integrity Constraints
• The Mechanisms of Constraint Checking
• Deferred Constraint Checking
• Constraint States

• 数据完整性简介
• 完整性约束概述
• 完整性约束的类型
• 约束检查的机制
• 延迟约束检查
• 约束状态

Introduction to Data Integrity

21.1 数据完整性简介

Figure 21-1 shows tables DEPT and EMP. Table DEPT has three columns:

• DEPTNO
• DNAME
• LOC

Each value in the DNAME column must be unique.

Table EMP has six columns:

• EMPNO
• ENAME
• ... Other Columns ...
• SAL
• COMM
• DEPTNO

Each row must have a value for the ENAME column. Each row must have a value for the EMPNO column, and the value must be unique. Each value in the DEPTNO column must match a value in the DEPTNO column of the DEPT table. Each value in the SAL column must be lower than 10,000.

图 21-1 定义了 DEPT 及 EMP 两个表。DEPT 表包含 3 列：

• DEPTNO
• DNAME
• LOC

DNAME 列的值必须唯一。

EMP 表包含 6 列：

• EMPNO
• ENAME
• ... Other Columns ...
• SAL
• COMM
• DEPTNO

每行的 ENAME 列必须有值。每行的 EMPNO 列必须有值，且值必须唯一。DEPTNO 列的值必须能与 DEPT 表的 DEPTNO 列的某个值相匹配。SAL 列的值不能超过 10,000。

Types of Data Integrity

21.1.1 数据完整性的类型

Null Rule

21.1.1.1 空规则

Unique Column Values

21.1.1.2 唯一列值

Primary Key Values

21.1.1.3 主键值

Referential Integrity Rules

21.1.1.4 引用完整性规则

• Restrict: Disallows the update or deletion of referenced data.
• Set to Null: When referenced data is updated or deleted, all associated dependent data is set to NULL.
• Set to Default: When referenced data is updated or deleted, all associated dependent data is set to a default value.
• Cascade: When referenced data is updated, all associated dependent data is correspondingly updated. When a referenced row is deleted, all associated dependent rows are deleted.
• No Action: Disallows the update or deletion of referenced data. This differs from RESTRICT in that it is checked at the end of the statement, or at the end of the transaction if the constraint is deferred. (Oracle uses No Action as its default action.)

• 限制（Restrict）：不允许对引用值进行更新与删除。
• 置空（Set to Null）：当因引用值被更新或删除后，所有受影响的依赖值都将被置为 NULL。
• 置默认值（Set to default）：当引用值被更新或删除后，所有受影响的依赖值都将被赋予一个默认值。
• 串联操作（Cascade）: 当引用值被更新后，所有受影响的依赖值也将被更新为相同的值。当引用数据行（referenced row）被删除后，所有受影响的依赖数据行（dependent row）也将被删除。
• 无操作（No Action）: 不允许对引用值进行更新与删除。此规则与 RESTRICT 有所不同，她只在语句结束时进行检查，如约束被延迟（deferred）则在事物结束时进行检查。（Oracle 的默认操作为无操作。）

Complex Integrity Checking

21.1.1.5 复杂完整性检查

How Oracle Enforces Data Integrity

21.1.2 Oracle 如何强制实现数据完整性

Integrity Constraints Description

21.1.2.1 完整性约束描述

• NOT NULL constraints for the rules associated with nulls in a column
• UNIQUE key constraints for the rule associated with unique column values
• PRIMARY KEY constraints for the rule associated with primary identification values
• FOREIGN KEY constraints for the rules associated with referential integrity. Oracle supports the use of FOREIGN KEY integrity constraints to define the referential integrity actions, including:
  
  
  • Update and delete No Action
  • Delete CASCADE
  • Delete SET NULL
• CHECK constraints for complex integrity rules

• NOT NULL（非空）约束（constraint）是关于列中空值（null）的规则
• UNIQUE key（唯一键）约束是关于唯一列值（unique column value）的规则
• PRIMARY KEY（主键）约束是关于主标识值（primary identification value）的规则
• FOREIGN KEY（外键）约束是关于引用完整性（referential integrity）规则。用户可以在 FOREIGN KEY 完整性约束中定义用完整性操作（referential integrity action），这些操作包括：
  
  
  • 更新无操作（update No Action）及删除无操作（delete No Action）
  • 删除串联操作（Delete CASCADE）
  • 删除置空（Delete SET NULL）
• CHECK（检查）约束，强制实现复杂完整性规则（complex integrity rule）

Note:

You cannot enforce referential integrity using declarative integrity constraints if child and parent tables are on different nodes of a distributed database. However, you can enforce referential integrity in a distributed database using database triggers (see next section).

提示：

如果子表（child table）及父表（parent table）位于分布式数据库系统（distributed database）的不同节点（node）上，则无法通过声明的完整性约束（declarative integrity constraint）强制实现引用完整性（referential integrity）规则。此种情况下可以使用数据库触发器（详见下节）。

Database Triggers

21.1.2.2 数据库触发器

See Also:

Chapter 22, "Triggers" for examples of triggers used to enforce data integrity

另见：

第 22 章，“触发器” 了解如何使用触发器强制实现数据完整性

Overview of Integrity Constraints

21.2 完整性约束概述

Note:

Operations on views (and synonyms for tables) are subject to the integrity constraints defined on the underlying base tables.

提示：

对视图（及表的同义词（synonym））执行的操作需要遵从底层基表（base tables）上的完整性约束。

Advantages of Integrity Constraints

21.2.1 完整性约束的优势

• Enforcing business rules in the code of a database application
• Using stored procedures to completely control access to data
• Enforcing business rules with triggered stored database procedures

• 在数据库应用程序代码中强制实现业务规则（business rule）
• 在存储过程（stored procedure）内实现对数据存取的控制
• 使用触发执行的（triggered）存储过程强制实现业务规则

See Also:

Chapter 22, "Triggers"

另见：

第 22 章，“触发器”

Declarative Ease

21.2.1.1 声明即用

Centralized Rules

21.2.1.2 规则集中化

Maximum Application Development Productivity

21.2.1.3 应用程序开发效率最大化

Immediate User Feedback

21.2.1.4 即时向用户反馈

Superior Performance

21.2.1.5 性能优势

Flexibility for Data Loads and Identification of Integrity Violations

21.2.1.6 数据加载时的灵活性及对违反完整性的数据的识别

The Performance Cost of Integrity Constraints

21.2.2 完整性约束对性能的影响

Types of Integrity Constraints

21.3 完整性约束的类型

• NOT NULL Integrity Constraints
• UNIQUE Key Integrity Constraints
• PRIMARY KEY Integrity Constraints
• Referential Integrity Constraints
• CHECK Integrity Constraints

• NOT NULL 完整性约束
• UNIQUE 键完整性约束
• PRIMARY KEY 完整性约束
• 引用完整性约束
• CHECK 完整性约束

NOT NULL Integrity Constraints

21.3.1 NOT NULL 完整性约束

Figure 21-2 shows Table EMP, which has eight columns:

• EMPNO
• ENAME
• JOB
• MGR
• HIREDATE
• SAL
• COMM
• DEPTNO

The ENAME column has a NOT NULL Constraint--no row may contain a null value for this column. Column COMM has an absence of a NOT NULL Constraint--any row can contain null for this column.

图 21-2 显示了 EMP 表，此表由 8 列构成：

• EMPNO
• ENAME
• JOB
• MGR
• HIREDATE
• SAL
• COMM
• DEPTNO

在 ENAME 列上定义了一个 NOT NULL 约束--任何数据行此列都不能为空值。COMM 列上没有 NOT NULL 约束--任何数据行此列都可以为空值。

UNIQUE Key Integrity Constraints

21.3.2 UNIQUE 键完整性约束

Figure 21-3 shows a UNIQUE key constraint defined on the DNAME column of the dept table to disallow rows with duplicate department names. If a null value is entered for the DNAME column, then the row is allowed. However, if a NOT NULL constraint is also defined on the DNAME column, then the row is not allowed.

例如在 图 21-3 中，dept 表的 DNAME 列上定义了一 UNIQUE 键约束，不允许此表内存在重复的部门名称。用户向 DNAME 列中插入空值是被允许的。但如果 DNAME 列上还定义了 NOT NULL 约束，则上述数据也不允许被插入。

Unique Keys

21.3.2.1 唯一键

Figure 21-4 shows that the customer table has a UNIQUE key constraint defined on the composite unique key: the area and phone columns. No rows can duplicate a set of values in the key.

If a null value is entered in one of the columns, then it is allowed. However, if a NOT NULL constraint is also defined on the column, then the row is not allowed.

图 21-4 显示了 customer 表上定义的 UNIQUE 键约束使用了复合唯一键：其中包含 area 和 phone 两列。任意两行中键的值不能重复。

用户可以向列中输入空值。但是，如果列上还定义了 NOT NULL 约束，则不允许输入空值。

UNIQUE Key Constraints and Indexes

21.3.2.2 UNIQUE 键约束与索引

Note:

If compatibility is set to Oracle9i or higher, then the total size in bytes of a key value can be almost as large as a full block. In previous releases key size could not exceed approximately half the associated database's block size.

Note:

如果数据库的兼容性被设为 Oracle9i 或更高，一个键值（key value）的容量（以字节为单位）可以与一个数据块的容量相当。而在此前的数据库版本中，一个键值的容量不能超过数据块容量的一半。

Combine UNIQUE Key and NOT NULL Integrity Constraints

21.3.2.3 UNIQUE 键约束与 NOT NULL 约束结合使用

Note:

Because of the search mechanism for UNIQUE constraints on more than one column, you cannot have identical values in the non-null columns of a partially null composite UNIQUE key constraint.

提示：

在定义了复合 UNIQUE 键约束的键中，如果部分列为空值，则其余非空列的值不能重复，这是定义在多列上的 UNIQUE 键约束的搜索机制（search mechanism）规定的。

PRIMARY KEY Integrity Constraints

21.3.3 PRIMARY KEY 完整性约束

• No two rows of a table have duplicate values in the specified column or set of columns.
• The primary key columns do not allow nulls. That is, a value must exist for the primary key columns in each row.

• 任意两行数据的 PRIMARY KEY 约束列（一列或多列）不存在重复值。
• 主键列的值不为空。即用户必须为主键列输入值。

Primary Keys

21.3.3.1 主键

• Each row in the table can be uniquely identified
• No duplicate rows exist in the table

• 表内的每行数据可以被唯一确定
• 表内不存在重复的数据行

Figure 21-5 shows Table DEPT, which has three columns: DEPTNO, DNAME, and LOC. The DEPTNO column has a Primary Key--no row may duplicate a value in the key and no null values are allowed. This illustration shows that two rows cannot be inserted into Table DEPT because one duplicates an existing value in the primary key, while the other contains a null value for the primary key.

图 21-5 显示了 DEPT 表，表内包含 3 列：DEPTNO，DNAME，及 LOC。在 DEPTNO 列上定义了主键约束--此列不能有重复数据，且不能为空。图中还显示了两行因为违反主键约束而无法插入 DEPT 表的数据，一行数据的主键值与已有数据重复，另一行数据的主键列为空值。

PRIMARY KEY Constraints and Indexes

21.3.3.2 PRIMARY KEY 约束与索引

• A unique index on that column
• A NOT NULL constraint for that column

• 在此列上创建唯一索引（unique index）
• 在此列上定义 NOT NULL constraint（约束）

Referential Integrity Constraints

21.3.4 引用完整性约束

Figure 21-6 is described in the text preceding the image.

图 21-6 将在下文中说明。

Self-Referential Integrity Constraints

21.3.4.1 自引用完整性约束

Figure 21-7 is described in the text preceding the image.

图 21-7 将在下文中说明。

Nulls and Foreign Keys

21.3.4.2 空值与外键

Actions Defined by Referential Integrity Constraints

21.3.4.3 在引用完整性中定义的操作

Note:

Other referential actions not supported by FOREIGN KEY integrity constraints of Oracle can be enforced using database triggers.

See Chapter 22, "Triggers" for more information.

提示：

Oracle FOREIGN KEY 完整性约束定义中不支持的引用操作可以通过数据库触发器实现。

见第 22 章，“触发器”了解更多信息。

Delete No Action

21.3.4.3.1 禁止操作

Delete Cascade

21.3.4.3.2 串联删除

Delete Set Null

21.3.4.3.3 置空

DML Restrictions with Respect to Referential Actions

21.3.4.3.4 各引用操作对 DML 的限制

Concurrency Control, Indexes, and Foreign Keys

21.3.4.4 并发控制，索引，及外键

No Index on the Foreign Key

21.3.4.4.1 外键上无索引

Note:

Indexed foreign keys only cause a row share table lock (also sometimes called a subshare table lock, SS). This prevents other transactions from exclusive locking the whole table, but it does not block DML on the parent or the child table.

另见：

如果外键上定义了索引，则对父表的主键执行 DML 操作时只需获得子表上的行共享表级锁（row share table lock）（也称为 subshare table lock，SS）。此锁不允许其他事务排他地对子表加锁，但允许对父表及子表执行 DML 操作。

See Also:

"DML Locks"

另见：

“DML 锁”

Figure 21-8 shows a parent table and its child table. Rows 1 through 4 of the parent table are indexed on keys 1 through 4, respectively. The child table is not foreign-key indexed to the parent table. Row 3 in the parent table is updated and acquires an exclusive row lock. At the same time, the child table acquires a share lock on the whole table.

图 21-8 显示了一个父表及一个相关的子表。父表的键上定义了索引。子表的外键上没有定义索引。当父表的第 3 行被更新时，事务获得了父表上的排他行级锁，同时获得了子表上的共享表级锁。

Index on the Foreign Key

21.3.4.4.2 外键上有索引

Figure 21-9 shows a parent table and its child table. Rows 1 through 4 of the parent table are indexed on keys 1 through 4, respectively. Row 5 in the child table is updated and acquires an exclusive row lock (key 2).

图 21-9 显示了一个父表及一个相关的子表。父表及子表的键上均定义了索引。当子表的第 5 行被更新时，事务获得了索引上的排他行级锁。

CHECK Integrity Constraints

21.3.5 CHECK 完整性约束

The Check Condition

21.3.5.1 检查条件

• It must be a Boolean expression evaluated using the values in the row being inserted or updated, and
• It cannot contain subqueries; sequences; the SQL functions SYSDATE, UID, USER, or USERENV; or the pseudocolumns LEVEL or ROWNUM.

• 检查条件必须是用于评估被插入或被更新数据行内的值的布尔表达式（Boolean expression）。
• 检查条件中不能包含：子查询（subquery）；序列（sequence）；SYSDATE，UID，USER，及 USERENV 等 SQL 函数；LEVEL 或 ROWNUM 虚列（pseudocolumn）。

See Also:

Oracle Database Globalization Support Guide for more information on globalization support features

另见：

Oracle Database Globalization Support Guide 了解更多关于全球化支持特性的信息

Multiple CHECK Constraints

21.3.5.2 多重 CHECK 约束

The Mechanisms of Constraint Checking

21.4 约束检查的机制

• The emp table has been defined as in Figure 21-7.
• The self-referential constraint makes the entries in the mgr column dependent on the values of the empno column. For simplicity, the rest of this discussion addresses only the empno and mgr columns of the emp table.

• 定义如 图 21-7 所示的 emp 表。
• 在 emp 表上定义自引用约束（self-referential constraint），mgr 列的值依赖于 empno 列的值。为了简化示例，以下内容只针对 emp 表的 empno 及 mgr 列展开讨论。

• A null can be entered for the mgr column of the first row, assuming that the mgr column does not have a NOT NULL constraint defined on it. Because nulls are allowed in foreign keys, this row is inserted successfully into the table.
• The same value can be entered in both the empno and mgr columns. This case reveals that Oracle performs its constraint checking after the statement has been completely run. To allow a row to be entered with the same values in the parent key and the foreign key, Oracle must first run the statement (that is, insert the new row) and then check to see if any row in the table has an empno that corresponds to the new row's mgr.
• A multiple row INSERT statement, such as an INSERT statement with nested SELECT statement, can insert rows that reference one another. For example, the first row might have empno as 200 and mgr as 300, while the second row might have empno as 300 and mgr as 200.
  
  This case also shows that constraint checking is deferred until the complete execution of the statement. All rows are inserted first, then all rows are checked for constraint violations. You can also defer the checking of constraints until the end of the transaction.

• 如果 mgr 列上没有定义 NOT NULL 约束，可以在第一行的 mgr 列上输入一个空值。由于外键约束允许空值，所以此行能成功插入表中。
• 可以向第一行的 empno 及 mgr 列输入一个相同的值。此种情况说明 Oracle 是在语句运行完成后执行的约束检查（constraint checking）。如果在第一行的父键（parent key）及外键（foreign key）插入相同的值，Oracle 必须首先运行语句（即插入数据行），再检查此行数据的 mgr 列值是否能与此表内的某个 empno 列值相匹配。
• 执行一个多行的 INSERT 语句，例如与 SELECT 语句结合的 INSERT 语句，将插入存在相互引用关系的多行数据。例如，第一行的 empno 列值为 200，mgr 列值为 300，而第二行的 empno 列值为 300，mgr 列值为 200。
  
  此种情况也说明 Oracle 将约束检查延迟（defer）直至语句运行结束。所有数据行首先被插入，之后 Oracle 逐行检查是否存在违反约束的情况。用户也可设置将约束检查延迟至事务（transaction）结束后。

Figure 21-10 shows the emp table with 2 columns: empno and mgr. The empno column has 3 rows: 210, 211, and 212. The mgr column has 2 rows: 210 and 211.

图 21-10 显示了 emp 表，其中包含两列：empno 与 mgr。empno 列有 3 个值：210，211，及 212。mgr 列有两个值：210 和 211。

UPDATE employees
SET employee_id = employee_id + 5000,
manager_id = manager_id + 5000;

UPDATE employees
SET employee_id = employee_id + 5000,
manager_id = manager_id + 5000;

Figure 21-11 shows the emp table with 2 columns: empno and mgr. The empno column has 3 rows: 210, 211, and 212. The mgr column has 2 rows: 210 and 211.

5000 is added to each employee number, and therefore each manager. In the first box, under the empno column, 210 is updated to 5210. In the second box, under the empno column, 211 is updated to 5211 and under the mgr column, 210 is updated to 5210. In the third box, under the empno column, 212 is updated to 5212 and under the mgr column, 211 is updated to 5211. Then, constraints are checked.

图 21-10 显示了 emp 表，其中包含两列：empno 与 mgr。empno 列有 3 个值：210，211，及 212。mgr 列有两个值：210 和 211。

首先为每个员工编号加 5000，再为每个经理编号加 5000。在第一步中，empno 列的 210 被更新为 5210。在第二步中，empno 列的 211 被更新为 5211，mgr 列的 210 被更新为 5210。在第三步中，empno 列的 212 被更新为 5212，mgr 列的 211 被更新为 5211。最后 Oracle 执行约束检查。

• NOT NULL
• UNIQUE key
• PRIMARY KEY
• All types of FOREIGN KEY constraints
• CHECK constraints

• NOT NULL
• UNIQUE 键
• PRIMARY KEY
• 所有类型的 FOREIGN KEY 约束
• CHECK 约束

See Also:

"Deferred Constraint Checking"

另见：

“延迟约束检查”

Default Column Values and Integrity Constraint Checking

21.4.1 列默认值与完整性约束检查

Deferred Constraint Checking

21.5 延迟约束检查

• A constraint is deferred if the system checks that it is satisfied only on commit. If a deferred constraint is violated, then commit causes the transaction to undo.
• If a constraint is immediate (not deferred), then it is checked at the end of each statement. If it is violated, the statement is rolled back immediately.

• 如果 Oracle 在事务提交（commit）时才对约束执行检查，则称此约束是延迟的（deferred）。如果数据违反了延迟约束，提交操作将导致事务被回滚（undo）。
• 如果约束是即时的（immediate）（非延迟的），则此约束将在语句执行结束后进行检查。如果数据违反了延迟约束，语句将被立即回滚。

Constraint Attributes

21.5.1 约束属性

• DEFERRABLE or NOT DEFERRABLE
• INITIALLY DEFERRED or INITIALLY IMMEDIATE

• DEFERRABLE 或 NOT DEFERRABLE
• INITIALLY DEFERRED 或 INITIALLY IMMEDIATE

See Also:

• Oracle Database SQL Reference for information about constraint attributes and their default values
• "Constraint States"
• "Constraint State Modification"

另见：

• Oracle Database SQL Reference 了解约束属性及各属性的默认值
• “约束状态”
• “约束状态修改”

SET CONSTRAINTS Mode

21.5.2 SET CONSTRAINTS 语句

Unique Constraints and Indexes

21.5.3 唯一约束与索引

Constraint States

21.6 约束状态

• ENABLE ensures that all incoming data conforms to the constraint
• DISABLE allows incoming data, regardless of whether it conforms to the constraint
• VALIDATE ensures that existing data conforms to the constraint
• NOVALIDATE means that some existing data may not conform to the constraint

• ENABLE（启用）确保所有输入的数据都遵从约束（constraint）
• DISABLE（禁用）总是允许输入数据，无论数据是否遵从约束
• VALIDATE（验证）确保已存在的数据遵从约束
• NOVALIDATE（无验证）允许已存在的数据不遵从约束

• ENABLE VALIDATE is the same as ENABLE. The constraint is checked and is guaranteed to hold for all rows.
• ENABLE NOVALIDATE means that the constraint is checked, but it does not have to be true for all rows. This allows existing rows to violate the constraint, while ensuring that all new or modified rows are valid.
  
  In an ALTER TABLE statement, ENABLE NOVALIDATE resumes constraint checking on disabled constraints without first validating all data in the table.
• DISABLE NOVALIDATE is the same as DISABLE. The constraint is not checked and is not necessarily true.
• DISABLE VALIDATE disables the constraint, drops the index on the constraint, and disallows any modification of the constrained columns.
  
  For a UNIQUE constraint, the DISABLE VALIDATE state enables you to load data efficiently from a nonpartitioned table into a partitioned table using the EXCHANGE PARTITION clause of the ALTER TABLE statement.

• ENABLE VALIDATE 与 ENABLE 相同。Oracle 将检查约束，并保证所有数据均遵从约束。
• ENABLE NOVALIDATE 表示 Oracle 将检查约束，但不保证所有数据均遵从约束。这确保了所有新插入或被修改的数据遵从约束，但允许已存在的数据不遵从约束。
  
  在 ALTER TABLE 语句中，使用 ENABLE NOVALIDATE 子句将恢复约束检查，但不会验证表中已存在的数据。
• DISABLE NOVALIDATE 与 DISABLE 相同。Oracle 不会检查约束，也不保证所有数据均遵从约束。
• DISABLE VALIDATE 将禁用约束，移除约束使用的索引，并禁止修改约束键的数据。
  
  用户在使用 ALTER TABLE 语句的 EXCHANGE PARTITION 子句将非分区表（nonpartitioned table）数据加载到分区表（partitioned table）时，如果存在 UNIQUE 约束，用户可以使用 DISABLE VALIDATE 来提高数据加载的效率。

• ENABLE implies VALIDATE, unless NOVALIDATE is specified.
• DISABLE implies NOVALIDATE, unless VALIDATE is specified.
• VALIDATE and NOVALIDATE do not have any default implications for the ENABLE and DISABLE states.
• When a unique or primary key moves from the DISABLE state to the ENABLE state, if there is no existing index, a unique index is automatically created. Similarly, when a unique or primary key moves from ENABLE to DISABLE and it is enabled with a unique index, the unique index is dropped.
• When any constraint is moved from the NOVALIDATE state to the VALIDATE state, all data must be checked. (This can be very slow.) However, moving from VALIDATE to NOVALIDATE simply forgets that the data was ever checked.
• Moving a single constraint from the ENABLE NOVALIDATE state to the ENABLE VALIDATE state does not block reads, writes, or other DDL statements. It can be done in parallel.

• ENABLE 即隐含着 VALIDATE，除非设定了 NOVALIDATE。
• DISABLE 即隐含着 NOVALIDATE，除非设定了 VALIDATE。
• VALIDATE 与 NOVALIDATE 默认不隐含 ENABLE 与 DISABLE。
• 当一个唯一键约束（unique key）或主键约束（primary key）从 DISABLE 转换为 ENABLE 状态时，如果没有索引存在，Oracle 将自动地创建一个唯一索引（unique index）。类似地，当一个唯一键约束或主键约束从 ENABLE 转换为 DISABLE 状态时，如果约束是通过唯一索引实现的，那么此索引将被移除。
• 任意约束从 NOVALIDATE 转换为 VALIDATE 状态时，所有数据均需进行约束检查。（此过程可能很慢。）将约束从 VALIDATE 转换为 NOVALIDATE 状态时无需任何检查。
• 将一个约束从 ENABLE NOVALIDATE 转换为 ENABLE VALIDATE 状态时，转换语句不会阻塞（block）读取，写入，或其他 DDL 语句。转换操作可以被并行执行。

See Also:

Oracle Database Administrator's Guide

另见：

Oracle Database Administrator's Guide

Constraint State Modification

21.6.1 约束状态修改

• DEFERRABLE or NOT DEFERRABLE
• INITIALLY DEFERRED or INITIALLY IMMEDIATE
• RELY or NORELY
• USING INDEX ...
• ENABLE or DISABLE
• VALIDATE or NOVALIDATE
• EXCEPTIONS INTO ...

• DEFERRABLE 或 NOT DEFERRABLE
• INITIALLY DEFERRED 或 INITIALLY IMMEDIATE
• RELY 或 NORELY
• USING INDEX ...
• ENABLE 或 DISABLE
• VALIDATE 或 NOVALIDATE
• EXCEPTIONS INTO ...

See Also:

Oracle Database SQL Reference for information about these constraint states

另见：

Oracle Database SQL Reference 了解关于约束状态的信息