Postgresql 中文操作指南
Synopsis
CREATE DOMAIN name [ AS ] data_type
[ COLLATE collation ]
[ DEFAULT expression ]
[ constraint [ ... ] ]
where constraint is:
[ CONSTRAINT constraint_name ]
{ NOT NULL | NULL | CHECK (expression) }Description
CREATE DOMAIN 创建一个新域。域本质上是一种数据类型，具有可选约束（对一系列允许值的限制）。定义域的用户将成为其所有者。
CREATE DOMAIN creates a new domain. A domain is essentially a data type with optional constraints (restrictions on the allowed set of values). The user who defines a domain becomes its owner.
如果给定了架构名称 (例如 CREATE DOMAIN myschema.mydomain … )，则该域将在指定的架构中创建。否则，将在当前架构中创建。域名称在其架构中现有的类型和域中必须是唯一的。
If a schema name is given (for example, CREATE DOMAIN myschema.mydomain …) then the domain is created in the specified schema. Otherwise it is created in the current schema. The domain name must be unique among the types and domains existing in its schema.
域对于将对字段的通用约束抽象到一个位置以进行维护非常有用。例如，几个表可能包含电子邮件地址列，都需要相同的 CHECK 约束来验证地址语法。定义域，而不是单独设置每个表的约束。
Domains are useful for abstracting common constraints on fields into a single location for maintenance. For example, several tables might contain email address columns, all requiring the same CHECK constraint to verify the address syntax. Define a domain rather than setting up each table’s constraint individually.
要创建域，你必须对基础类型具有 USAGE 权限。
To be able to create a domain, you must have USAGE privilege on the underlying type.
Parameters
-
name
-
The name (optionally schema-qualified) of a domain to be created.
-
-
data_type
-
The underlying data type of the domain. This can include array specifiers.
-
-
collation
-
An optional collation for the domain. If no collation is specified, the domain has the same collation behavior as its underlying data type. The underlying type must be collatable if COLLATE is specified.
-
-
DEFAULT _expression_
-
The DEFAULT clause specifies a default value for columns of the domain data type. The value is any variable-free expression (but subqueries are not allowed). The data type of the default expression must match the data type of the domain. If no default value is specified, then the default value is the null value.
-
The default expression will be used in any insert operation that does not specify a value for the column. If a default value is defined for a particular column, it overrides any default associated with the domain. In turn, the domain default overrides any default value associated with the underlying data type.
-
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CONSTRAINT _constraint_name_
-
An optional name for a constraint. If not specified, the system generates a name.
-
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NOT NULL
-
Values of this domain are prevented from being null (but see notes below).
-
-
NULL
-
Values of this domain are allowed to be null. This is the default.
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This clause is only intended for compatibility with nonstandard SQL databases. Its use is discouraged in new applications.
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-
CHECK (_expression)_
-
CHECK clauses specify integrity constraints or tests which values of the domain must satisfy. Each constraint must be an expression producing a Boolean result. It should use the key word VALUE to refer to the value being tested. Expressions evaluating to TRUE or UNKNOWN succeed. If the expression produces a FALSE result, an error is reported and the value is not allowed to be converted to the domain type.
-
Currently, CHECK expressions cannot contain subqueries nor refer to variables other than VALUE.
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When a domain has multiple CHECK constraints, they will be tested in alphabetical order by name. (PostgreSQL versions before 9.5 did not honor any particular firing order for CHECK constraints.)
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Notes
域约束（尤其是 NOT NULL ）会在将值转换为域类型时得到检查。尽管存在此类约束，但名义上属于该域类型的列仍可能读为 null。例如，如果域列位于外连接的可空一侧，则这可能会在外连接查询中发生。一个更微妙的示例是
Domain constraints, particularly NOT NULL, are checked when converting a value to the domain type. It is possible for a column that is nominally of the domain type to read as null despite there being such a constraint. For example, this can happen in an outer-join query, if the domain column is on the nullable side of the outer join. A more subtle example is
INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM tab WHERE false));空的标量子选择将生成一个 null 值，该值被认为属于该域类型，因此不会对此值应用进一步的约束检查，并且插入操作将成功。
The empty scalar sub-SELECT will produce a null value that is considered to be of the domain type, so no further constraint checking is applied to it, and the insertion will succeed.
由于 SQL 通常假设 null 值是每种数据类型的有效值，因此很难避免此类问题。因此，最佳做法是设计该域的约束，以便允许 null 值，然后根据需要将列 NOT NULL 约束应用到该域类型的列，而不是直接应用到该域类型上。
It is very difficult to avoid such problems, because of SQL’s general assumption that a null value is a valid value of every data type. Best practice therefore is to design a domain’s constraints so that a null value is allowed, and then to apply column NOT NULL constraints to columns of the domain type as needed, rather than directly to the domain type.
PostgreSQL 假设 CHECK 约束的条件是不可变的，换句话说，它们将始终针对相同的输入值给出相同的结果。此假设论证了只能在首次将值转换为域类型时检查 CHECK 约束，而不能在其他时间进行检查。（这在本质上与表 CHECK 约束的处理相同，如 Section 5.4.1 中所述。）
PostgreSQL assumes that CHECK constraints' conditions are immutable, that is, they will always give the same result for the same input value. This assumption is what justifies examining CHECK constraints only when a value is first converted to be of a domain type, and not at other times. (This is essentially the same as the treatment of table CHECK constraints, as described in Section 5.4.1.)
以在 CHECK 表达式中引用用户定义函数，然后更改该函数的行为为例来打破此假设的做法很常见。PostgreSQL 不会禁止这样做，但如果没有违反 CHECK 约束的存储域类型值，它不会注意到。这会导致随后的数据库转储和还原失败。处理此类更改的建议方法是删除约束（使用 ALTER DOMAIN ）、调整函数定义，然后重新添加约束，从而根据存储的数据重新对它进行检查。
An example of a common way to break this assumption is to reference a user-defined function in a CHECK expression, and then change the behavior of that function. PostgreSQL does not disallow that, but it will not notice if there are stored values of the domain type that now violate the CHECK constraint. That would cause a subsequent database dump and restore to fail. The recommended way to handle such a change is to drop the constraint (using ALTER DOMAIN), adjust the function definition, and re-add the constraint, thereby rechecking it against stored data.
确保域 CHECK 表达式不会引发错误也是一种良好做法。
It’s also good practice to ensure that domain CHECK expressions will not throw errors.
Examples
此示例创建了 us_postal_code 数据类型，然后在表定义中使用此类型。使用正则表达式测试来验证该值是否像有效的美国邮政编码：
This example creates the us_postal_code data type and then uses the type in a table definition. A regular expression test is used to verify that the value looks like a valid US postal code:
CREATE DOMAIN us_postal_code AS TEXT
CHECK(
VALUE ~ '^\d{5}$'
OR VALUE ~ '^\d{5}-\d{4}$'
);
CREATE TABLE us_snail_addy (
address_id SERIAL PRIMARY KEY,
street1 TEXT NOT NULL,
street2 TEXT,
street3 TEXT,
city TEXT NOT NULL,
postal us_postal_code NOT NULL
);