NAME

ALTER DOMAIN - 改變一個(gè)域的定義

SYNOPSIS

ALTER DOMAIN name
    { SET DEFAULT expression | DROP DEFAULT }
ALTER DOMAIN name
    { SET | DROP } NOT NULL
ALTER DOMAIN name
    ADD domain_constraint
ALTER DOMAIN name
    DROP CONSTRAINT constraint_name [ RESTRICT | CASCADE ]
ALTER DOMAIN name
    OWNER TO new_owner 
  

DESCRIPTION 描述

ALTER DOMAIN 修改一個(gè)現(xiàn)有域的定義。 它有幾種子形式：

SET/DROP DEFAULT

 這些形式設(shè)置或者刪除一個(gè)域的缺省值。請注意缺省只適用于隨后的 INSERT 命令；他們并不影響使用該域已經(jīng)在表中的行。

SET/DROP NOT NULL

 這些形式改變一個(gè)域是否標(biāo)記為允許 NULL 值或者是拒絕 NULL 值。 在使用域的字段包含非空的值的時(shí)候，你只可以 SET NOT NULL。

ADD domain_constraint

 這種形式向域中增加一種新的約束，使用的語法和 CREATE DOMAIN [create_domain(7)] 一樣。這樣做只有在所有使用域的字段滿足新的約束的條件下才能成功。

DROP CONSTRAINT

 這種形式刪除一個(gè)域上的約束。

OWNER

 這種形式把域的所有者改變?yōu)榱硗庖粋€(gè)用戶。

 要使用 ALTER DOMAIN，你必須擁有該域；但是使用 ALTER DOMAIN OWNER 的時(shí)候你必須是數(shù)據(jù)庫超級(jí)用戶。

PARAMETERS 參數(shù)

name

 一個(gè)要修改的現(xiàn)有域的名字（可以有模式修飾）。

domain_constraint

 域的新的域約束。

constraint_name

 要?jiǎng)h除的現(xiàn)有約束。

CASCADE

 自動(dòng)刪除依賴這個(gè)對象的約束。

RESTRICT

 如果有任何依賴對象，則拒絕刪除約束。這是缺省行為。

new_owner

 域的新所有者的用戶名。

EXAMPLES 例子

 給一個(gè)域增加一個(gè) NOT NULL 約束：

ALTER DOMAIN zipcode SET NOT NULL;
  

 從一個(gè)域里刪除一個(gè) NOT NULL 約束：

ALTER DOMAIN zipcode DROP NOT NULL;

 給一個(gè)域里增加一個(gè)檢查約束：

ALTER DOMAIN zipcode ADD CONSTRAINT zipchk CHECK (char_length(VALUE) = 5);
   

 從一個(gè)域里刪除一個(gè)檢查約束：

ALTER DOMAIN zipcode DROP CONSTRAINT zipchk;

#p#

NAME

ALTER DOMAIN - change the definition of a domain

SYNOPSIS

ALTER DOMAIN name
    { SET DEFAULT expression | DROP DEFAULT }
ALTER DOMAIN name
    { SET | DROP } NOT NULL
ALTER DOMAIN name
    ADD domain_constraint
ALTER DOMAIN name
    DROP CONSTRAINT constraint_name [ RESTRICT | CASCADE ]
ALTER DOMAIN name
    OWNER TO new_owner 
  

DESCRIPTION

ALTER DOMAIN changes the definition of an existing domain. There are several sub-forms:

SET/DROP DEFAULT

These forms set or remove the default value for a domain. Note that defaults only apply to subsequent INSERT commands; they do not affect rows already in a table using the domain.

SET/DROP NOT NULL

These forms change whether a domain is marked to allow NULL values or to reject NULL values. You may only SET NOT NULL when the columns using the domain contain no null values.

ADD domain_constraint

This form adds a new constraint to a domain using the same syntax as CREATE DOMAIN [create_domain(7)]. This will only succeed if all columns using the domain satisfy the new constraint.

DROP CONSTRAINT

This form drops constraints on a domain.

OWNER

This form changes the owner of the domain to the specified user.

You must own the domain to use ALTER DOMAIN; except for ALTER DOMAIN OWNER, which may only be executed by a superuser.

PARAMETERS

name

The name (possibly schema-qualified) of an existing domain to alter.

domain_constraint

New domain constraint for the domain.

constraint_name

Name of an existing constraint to drop.

CASCADE

Automatically drop objects that depend on the constraint.

RESTRICT

Refuse to drop the constraint if there are any dependent objects. This is the default behavior.

new_owner

The user name of the new owner of the domain.

EXAMPLES

To add a NOT NULL constraint to a domain:

ALTER DOMAIN zipcode SET NOT NULL;
   

To remove a NOT NULL constraint from a domain:

ALTER DOMAIN zipcode DROP NOT NULL;
   

To add a check constraint to a domain:

ALTER DOMAIN zipcode ADD CONSTRAINT zipchk CHECK (char_length(VALUE) = 5);
   

To remove a check constraint from a domain:

ALTER DOMAIN zipcode DROP CONSTRAINT zipchk;