mysql查询字段唯一约束条件

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL UNIQUE,
    email VARCHAR(100) NOT NULL UNIQUE
);

INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com');
INSERT INTO users (username, email) VALUES ('jane_doe', 'jane@example.com');

SELECT * FROM users WHERE username = 'john_doe';

INSERT IGNORE INTO users (username, email) VALUES ('john_doe', 'john@example.com');

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

SELECT COUNT(*) FROM users WHERE username = 'john_doe';

SELECT EXISTS(SELECT 1 FROM users WHERE username = 'john_doe');