SQL UNIQUE 关键字
UNIQUE
The UNIQUE constraint ensures that all values in a column are unique。
SQL UNIQUE Constraint on CREATE TABLE
The following SQL creates a UNIQUE constraint on the "ID" column when the "Persons" table is created
SQL Server / Oracle / MS Access
CREATE TABLE Persons (
ID int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int
);
MySQL
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
UNIQUE (ID)
);To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax
MySQL / SQL Server / Oracle / MS Access
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
CONSTRAINT UC_Person UNIQUE (ID,LastName)
);SQL UNIQUE Constraint on ALTER TABLE
To create a UNIQUE constraint on the "ID" column when the table is already created, use the following SQL
MySQL / SQL Server / Oracle / MS Access
ALTER TABLE Persons
ADD UNIQUE (ID);To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax
MySQL / SQL Server / Oracle / MS Access
ALTER TABLE Persons
ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);DROP a UNIQUE Constraint (删除 UNIQUE 约束)
To drop a UNIQUE constraint, use the following SQL (要删除 UNIQUE 约束，请使用以下 SQL 语句)
MySQL
ALTER TABLE Persons
DROP INDEX UC_Person;
SQL Server / Oracle / MS Access
ALTER TABLE Persons
DROP CONSTRAINT UC_Person;
