mysql中主键约束

INSERT INTO users (id, name, email) VALUES (1, 'Alice', 'alice@example.com');

ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

INSERT INTO users (id, name, email)
SELECT 1, 'Alice', 'alice@example.com'
WHERE NOT EXISTS (SELECT 1 FROM users WHERE id = 1);

CREATE TABLE users (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    email VARCHAR(50)
);

INSERT INTO users (id, name, email) VALUES (NULL, 'Alice', 'alice@example.com');

ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: YES)

INSERT INTO users (id, name, email) VALUES (1, 'Alice', 'alice@example.com');