增加约束键
ALTER TABLE Persons ADD UNIQUE (P_Id)
创建表时加入约束键
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id),
UNIQUE (LastName)
)
ALTER TABLE Persons ADD UNIQUE (P_Id)
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id),
UNIQUE (LastName)
)