PostgreSQL-constraint

CREATE TABLE语法
CREATE [[ GLOBAL | LOCAL ]{ TEMPORARY | TEMP }] TABLE table_name 
(
{ column_name data_type [ DEFAULT default_expr ][ column_constraint [...]]
| table_constraint
| LIKE parent_table [{ INCLUDING | EXCLUDING } DEFAULTS ]
}[,...]
)
[ INHERITS ( parent_table [,...])]
[ WITH OIDS | WITHOUT OIDS ]
[ ON COMMIT { PRESERVE ROWS | DELETE ROWS | DROP }]
[ TABLESPACE tablespace ]
 

紧跟在字段定义后 column_constraint	和字段定义并列写在表定义中（约束子句） table_constraint
[CONSTRAINT constraint_name] CHECK (expression)	[CONSTRAINT constraint_name] CHECK ( expression )	表约 束	检查
NOT NULL | NULL		字段 约束 （修 饰词）	非空
[CONSTRAINT constraint_name] CHECK (colunmn is not null)	[CONSTRAINT constraint_name] CHECK (colunmn is not null)	表约 束	非空
UNIQUE [USING INDEX TABLESPACE tablespace]	[CONSTRAINT constraint_name] UNIQUE (column_name [,...]) [USING INDEX TABLESPACE tablespace]	表索 引约 束	惟一
[CONSTRAINT constraint_name] REFERENCES ref_table [( ref_column )] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] [ ON DELETE action ][ ON UPDATE action ]	[CONSTRAINT constraint_name] FOREIGN KEY ( column_name [,...]) REFERENCES ref_table [( ref_column [,...])] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] [ ON DELETE action ][ ON UPDATE action ]	表外 键	外键
ADD [ COLUMN ] column type [ column_constraint [...]] ALTER [ COLUMN ] column SET DEFAULT expression ALTER [ COLUMN ] column DROP DEFAULT ALTER [ COLUMN ] column { SET | DROP } NOT NULL	ADD table_constraint DROP CONSTRAINT constraint_name [ RESTRICT | CASCADE ] CLUSTER ON index_name	相关 DDL

非空约束not null有两种方式实现,一种是修饰词“非空”,如 
creata table a(a int not null);
alter table a alter a drop not null;

一种是check，而check又分为字段约束和表约束，如
create table a(a int check(a is not null));
alter table a drop constraint a_a_check;

默认值default 也是经过alter语句的set和drop来进行修改的。

惟一约束unique 能够经过DROP CONSTRAINT 约束名称来删除。

在使用alter table 语句添加新字段的时候只能使用column_constraint，只有单纯添加约束才能使用table_constraint，两种状况下unique约束就存在区别。