mysql如何加唯一约束条件

ALTER TABLE table_name
ADD UNIQUE (column1, column2, ...);

ALTER TABLE users
ADD UNIQUE (username);

CREATE TABLE table_name (
    column1 datatype,
    column2 datatype,
    ...
    UNIQUE (column1, column2, ...)
);

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50),
    email VARCHAR(100),
    UNIQUE (username)
);

INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com');
INSERT INTO users (username, email) VALUES ('john_doe', 'johndoe@example.com'); -- 这将导致错误

INSERT INTO users (username, email) VALUES ('john_doe', 'johndoe@example.com')
ON DUPLICATE KEY UPDATE email = 'johndoe@example.com';

ALTER TABLE table_name
DROP INDEX unique_constraint_name;

ALTER TABLE users
DROP INDEX username;