(1)創建表
create table userinfos (
?? user_id int8,
?? username varchar(40),
?? password varchar(60)
)

(2)添加逐漸約束

?alter table userinfs add primary key (user_id);

(3)創建表的時候直接指定主鍵
create table userinfos (
?? user_id int8 primary key,
?? username varchar(40),
?? password varchar(60)
)

(4) 創建表orders

create table orders (
? order_id int8 primary key,
? user_id int8,
? order_date date
)

(5)創建外建

alter table orders add foreign key (user_id) reference userinfos(user_id);

(6)創建表的時候指定外建

create table orders (
? order_id int8 primary key,
? user_id int8 references userinfos(user_id),
? order_date date
)

(7)刪除約束
查看約束的名字 ，比如userinfos_pkey
alter table userinfs drop constraint "userinfos_pkey"
?









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