Types of Integrity Constraints
Oracle Database enables you to apply constraints both at the table and column level.
A constraint specified as part of the definition of a column or attribute is an inline specification. A constraint specified as part of the table definition is an out-of-line specification.
A key is the column or set of columns included in the definition of certain types of integrity constraints. Keys describe the relationships between the tables and columns of a relational database. Individual values in a key are called key values.
The following table describes the types of constraints. Each can be specified either inline or out-of-line, except for NOT NULL, which must be inline.
完整性(Integrity)约束(Constraint)的类型
Oracle数据库允许你在表级和列级应用约束
作为列或属性定义(definition)的一部分的约束是内联规范(specification)。作为表定义的一部分的约束是越界规范（out-of-line不知道怎么解释）。
包含在明确完整性约束中的单列或多列是一个键。在关系型数据库中多个键描述表和列之间的关系。在一个键中的单独的值（可能为一列，可能为多列组合）叫做键值。
下面的表格介绍约束的类型。每一中约束都可以被指定(specified)为内联或脱机规范，除了NOT NULL约束。NOT NULL约束只能符合内联规范。
Table 5-1 Types of Integrity Constraints
| Constraint Type |
Description |
See Also |
| NOT NULL |
Allows or disallows inserts or updates of rows containing a null in a specified column. |
|
| Unique key |
Prohibits multiple rows from having the same value in the same column or combination of columns but allows some values to be null. |
|
| Primary key |
Combines a NOT NULL constraint and a unique constraint. It prohibits multiple rows from having the same value in the same column or combination of columns and prohibits values from being null. |
|
| Foreign key |
Designates a column as the foreign key and establishes a relationship between the foreign key and a primary or unique key, called the referenced key. |
|
| Check |
Requires a database value to obey a specified condition. |
|
| REF |
Dictates types of data manipulation allowed on values in a REF column and how these actions affect dependent values. In an object-relational database, a built-in data type called a REF encapsulates a reference to a row object of a specified object type. Referential integrity constraints on REF columns ensure that there is a row object for the REF. |
Oracle Database Object-Relational Developer's Guide to learn about REFconstraints |
NOT NULL约束：非空约束
允许或不允许在指定列中插入或更新包含空值的行
Unique Key约束：唯一约束
禁止(Prohibits) 多个(倍数multiple)在同一个列中或多个结合(combination)的列中的行具有相同的值 ，但是允许一些为空的值（即空值无法做判断，NULL = NULL 是伪命题）
Primary Key约束：主键约束
结合非空约束和唯一键约束。主键禁止一个或多个结合的列中具有相同的值，并且禁止空值
Foreign Key约束：外键约束
指定(Designates)一个列作为外键 并且建立(establishes)一个关系 在外键和一个主键或唯一键（被称为被引用键）
将列指定为外键，并和被称为被引用键的主键或唯一键之间建立关系
Check约束：检查约束
要求数据库的值遵从(obey)一个特殊的条件
REF约束：
描述引用列(REF column)中的值允许的数据操作(manipulation)类型以及这些操作如何影响依赖的数据的。
在一个对象-关系型数据库中，一个被称为REF的内建的数据类型，封装(encapsulates)了一个对(指定对象类型的)行对象的引用。
引用列上的完整性约束确保了引用有一个行对象。
See Also:
- Oracle Database SQL Language Reference to learn more about the types of constraints
NOT NULL Integrity Constraints
A NOT NULL constraint requires that a column of a table contain no null values. A null is the absence of a value. By default, all columns in a table allow nulls.
NOT NULL constraints are intended for columns that must not lack values. For example, the hr.employees table requires a value in the email column. An attempt to insert an employee row without an email address generates an error:
SQL> INSERT INTO hr.employees (employee_id, last_name) values (999, 'Smith');
ERROR at line 1:
ORA-01400: cannot insert NULL into ("HR"."EMPLOYEES"."EMAIL")
You can only add a column with a NOT NULL constraint if the table does not contain any rows or if you specify a default value.
NOT NULL完整性约束
一个NOT NULL约束要求表中包含的列不能有空值。null是缺少值。默认情况下，表中所有的列都允许空值。
NOT NULL约束被预期(intended)/用于 不能缺少(lack)值的列。例如，hr.employees表要求一个email列值。我们尝试(attempt)往employee表中添加数据却不添加email地址，生成一个错误：
ORA-01400：不能将NULL插入HR.EMPLOYEES表的EMAIL列。
You can only add a column with a NOT NULL constraint if the table does not contain any rows or if you specify a default value.
这段话不知道该怎么理解。。。
Unique Constraints
A unique key constraint requires that every value in a column or set of columns be unique. No rows of a table may have duplicate values in a single column (the unique key) or set of columns (the composite unique key) with a unique key constraint.
Note:
The term key refers only to the columns defined in the integrity constraint. Because the database enforces a unique constraint by implicitly creating or reusing an index on the key columns, the term unique key is sometimes incorrectly used as a synonym for unique key constraint or unique index.
Unique key constraints are appropriate for any column where duplicate values are not allowed. Unique constraints differ from primary key constraints, whose purpose is to identify each table row uniquely, and typically contain values that have no significance other than being unique. Examples of unique keys include:
- A customer phone number, where the primary key is the customer number
- A department name, where the primary key is the department number
As shown in Example 2-1, a unique key constraint exists on the email column of the hr.employees table. The relevant part of the statement is as follows:
CREATE TABLE employees ( ...
, email VARCHAR2(25)
CONSTRAINT emp_email_nn NOT NULL ...
, CONSTRAINT emp_email_uk UNIQUE (email) ... );
The emp_email_uk constraint ensures that no two employees have the same email address, as shown in the following example:
SQL> SELECT employee_id, last_name, email FROM employees WHERE email = 'PFAY';
EMPLOYEE_ID LAST_NAME EMAIL
----------- ------------------------- -------------------------
202 Fay PFAY
SQL> INSERT INTO employees (employee_id, last_name, email, hire_date, job_id)
1 VALUES (999,'Fay','PFAY',SYSDATE,'ST_CLERK');
.
.
.
ERROR at line 1:
ORA-00001: unique constraint (HR.EMP_EMAIL_UK) violated
Unless a NOT NULL constraint is also defined, a null always satisfies a unique key constraint. Thus, columns with both unique key constraints and NOT NULL constraints are typical. This combination forces the user to enter values in the unique key and eliminates the possibility that new row data conflicts with existing row data.
Note:
Because of the search mechanism for unique key constraints on multiple columns, you cannot have identical values in the non-null columns of a partially null composite unique key constraint.
Example 5-1 Unique Constraint
SQL> SELECT employee_id, last_name, email FROM employees WHERE email = 'PFAY';
EMPLOYEE_ID LAST_NAME EMAIL
----------- ------------------------- -------------------------
202 Fay PFAY
SQL> INSERT INTO employees (employee_id, last_name, email, hire_date, job_id)
1 VALUES (999,'Fay','PFAY',SYSDATE,'ST_CLERK');
.
.
.
ERROR at line 1:
ORA-00001: unique constraint (HR.EMP_EMAIL_UK) violated
See Also:
- Oracle Database 2 Day Developer's Guide for examples of adding UNIQUE constraints to a table
唯一约束
唯一键约束，要求一列或多列中的每个值都必须是唯一的。
表中的任何行，在具有唯一性约束的单列（唯一键）或一组列（复合（composite）唯一键）中都不可能出现重复值。
Note：
术语"键"只指完整性约束中定义的列。因为数据库在键列上通过隐式创建或重用索引来强制执行唯一约束，所以有时错误的将术语"唯一键"用作"唯一键约束"或"唯一索引"的同义词。
（理解："唯一键"是一列或一组列，而不是"唯一键约束"或"唯一索引"
"唯一键约束"是要求一列或多列中的每个值都必须是唯一的，是对值的要求/约束
"唯一索引"是？？？ 下面有链接进行解释）
唯一键约束适用于不允许值重复的任何列。唯一性约束和主键约束不同，主键约束的目的是唯一地标识每个表行，并且通常包含除了唯一没有意义的值。（除了是唯一的，并没有其它特殊意义）如下是唯一键的示例：
一个客户的电话号码，主键就是客户号
一个部门的名称，主键就是部门
如示例2-1所示，在hr.employees表的eamil列上存在唯一键约束。声明的相关部分如下：
CREATE TABLE employees( ...
, email VARCHAR2(25)
CONSTRAINT emp_email_nn NOT NULL ...
, CONSTRAINT emp_email_uk UNIQUE(email) ...
);
约束emp_email_uk确保了没有两个雇员拥有同一个邮箱地址，如下例所示：
SQL> SELECT employee_id, last_name, email FROM employees WHERE email='PFAY';
值如下：202 Fay PFAY
SQL> INSERT INTO employees(employee_id, last_name, email)
VALUES (999,'Fay','PFAY');
ERROR at line 1：
ORA-00001：unique constraint (HR.EMP_EMAIL_UK) violated
除非定义了一个非空约束，否则空值始终满足唯一键约束。因此，具有唯一键约束和非空约束的列是典型的。这种组合强制要求用户在唯一键中输入值，并消除了新行数据与现有行数据冲突的可能性。
Note：
因为在多个列上搜索唯一键约束的机制，部分为空的组合唯一键约束的非空列中，你不能得到相同的值（
意思是这样的：1.多个列组合成唯一键约束 2.有的列允许为空 3.在其余非空列中，不存在完全相同的值）。
（这句话的意思是：如果多个列组成唯一键约束，那么这些列就是一个整体。比较唯一性时，要从整体/多个列进行比较）
See Also:
- "Unique and Nonunique Indexes" 这介绍了唯一索引和非唯一索引
- Oracle Database 2 Day Developer's Guide for examples of adding UNIQUE constraints to a table
