环境系统平台：microsoft windows (64-bit) 10版本：4.5
瀚高数据库中支持使用以下语句创建用户定义的数据类型：
CREATE DOMAIN：它创建了一个用户定义的数据类型，可以有可选的约束，基于其他基本类型，实质是定义一个域。
CREATE TYPE：它通常用于使用存储过程创建复合类型(两种或多种数据类型混合的数据类型)。
一、domain用法及示例
假如有以下表结构：
create table test_domain (id varchar,md5 text not null check(length(md5)=32));
其中md5列的类型及约束，可以定义一个domain来抽象，如下：
highgo=# create domain md5 ashighgo-# text not nullhighgo-# check (highgo(# length(value) = 32highgo(# );CREATE DOMAINhighgo=# highgo=# \dD md5 List of domains Schema | Name | Type | Collation | Nullable | Default | Check --------+------+------+-----------+----------+---------+---------------------------- public | md5 | text | | not null | | CHECK (length(VALUE) = 32)(1 row)highgo=# create table test_domain (id varchar,md5 md5);CREATE TABLEhighgo=# insert into test_domain values('1','2');ERROR: value for domain md5 violates check constraint "md5_check"highgo=# insert into test_domain values('2','76a2173be6393254e72ffa4d6df1030a');INSERT 0 1
二、创建MySQL中datetime类型
highgo=# create domain datetime as timestamp without time zone;highgo=# create table t_time (id int,create_time datetime);CREATE TABLEhighgo=# \d+ t_time Table "public.t_time" Column | Type | Collation | Nullable | Default | Storage | Stats target | Description -------------+----------+-----------+----------+---------+---------+--------------+------------- id | integer | | | | plain | | create_time | datetime | | | | plain | | Access method: heaphighgo=# insert into t_time values (1,now()),(2,now());INSERT 0 2highgo=# highgo=# select * from t_time; id | create_time ----+---------------------------- 1 | 2021-08-03 19:28:11.207324 2 | 2021-08-03 19:28:11.207324(2 rows)
三、create type用法及示例
CREATE TYPE name AS ( [ attribute_name data_type [ COLLATE collation ] [, ... ] ] )CREATE TYPE name AS ENUM ( [ 'label' [, ... ] ] )CREATE TYPE name AS RANGE ( SUBTYPE = subtype [ , SUBTYPE_OPCLASS = subtype_operator_class ] [ , COLLATION = collation ] [ , CANONICAL = canonical_function ] [ , SUBTYPE_DIFF = subtype_diff_function ])CREATE TYPE name ( INPUT = input_function, OUTPUT = output_function [ , RECEIVE = receive_function ] [ , SEND = send_function ] [ , TYPMOD_IN = type_modifier_input_function ] [ , TYPMOD_OUT = type_modifier_output_function ] [ , ANALYZE = analyze_function ] [ , INTERNALLENGTH = { internallength | VARIABLE } ] [ , PASSEDBYVALUE ] [ , ALIGNMENT = alignment ] [ , STORAGE = storage ] [ , LIKE = like_type ] [ , CATEGORY = category ] [ , PREFERRED = preferred ] [ , DEFAULT = default ] [ , ELEMENT = element ] [ , DELIMITER = delimiter ] [ , COLLATABLE = collatable ])CREATE TYPE name
创建示例：
CREATE TYPE compfoo AS (f1 int, f2 text);CREATE FUNCTION getfoo() RETURNS SETOF compfoo AS $$ SELECT fooid, fooname FROM foo$$ LANGUAGE SQL;CREATE TYPE bug_status AS ENUM ('new', 'open', 'closed');CREATE TABLE bug ( id serial, description text, status bug_status);CREATE TYPE float8_range AS RANGE (subtype = float8, subtype_diff = float8mi);
以上就是怎么创建一个创建MySQL数据库中的datetime类型的详细内容，更多请关注创想鸟其它相关文章！
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