PostgreSQL-constraint

CREATE TABLE語法
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 ]
 

緊跟在字段定義後 column_constraint	和字段定義並列寫在表定義中（約束子句） table_constraint
[CONSTRAINT constraint_name] CHECK (expression)	[CONSTRAINT constraint_name] CHECK ( expression )	表約 束	檢查
NOT NULL | NULL		字段 約束 （修 飾詞）	非空
[CONSTRAINT constraint_name] CHECK (colunmn is not null)	[CONSTRAINT constraint_name] CHECK (colunmn is not null)	表約 束	非空
UNIQUE [USING INDEX TABLESPACE tablespace]	[CONSTRAINT constraint_name] UNIQUE (column_name [,...]) [USING INDEX TABLESPACE tablespace]	表索 引約 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 鍵	外鍵
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	相關 DDL

非空約束not null有兩種方式實現,一種是修飾詞「非空」,如 
creata table a(a int not null);
alter table a alter a drop not null;

一種是check，而check又分爲字段約束和表約束，如
create table a(a int check(a is not null));
alter table a drop constraint a_a_check;

默認值default 也是經過alter語句的set和drop來進行修改的。

惟一約束unique 能夠經過DROP CONSTRAINT 約束名稱來刪除。

在使用alter table 語句添加新字段的時候只能使用column_constraint，只有單純添加約束才能使用table_constraint，兩種狀況下unique約束就存在區別。