MyBatis主键生成策略中useGeneratedKeys和＜selectKey＞的区别

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

public class User {
    private Integer id;
    private String username;
    private String email;

    // getters and setters...
}

<insert id="insertUser" useGeneratedKeys="true" keyProperty="id">
  INSERT INTO users (username, email)
  VALUES (#{username}, #{email})
</insert>

<insert id="batchInsertUsers" useGeneratedKeys="true" keyProperty="id" keyColumn="id">
  INSERT INTO users (username, email)
  <foreach collection="list" item="user" separator=",">
    (#{user.username}, #{user.email})
  </foreach>
</insert>

List<User> users = // ... your list of users to insert
SqlSession sqlSession = sqlSessionFactory.openSession();
try {
    UserMapper mapper = sqlSession.getMapper(UserMapper.class);
    for (User user : users) {
        mapper.insertUser(user);
        // MyBatis will automatically set the generated ID into the 'user' object
    }
    sqlSession.commit();
} finally {
    sqlSession.close();
}

try {
    sqlSession.insert("insertUser", user);
    sqlSession.commit();
} catch (PersistenceException e) {
    sqlSession.rollback();
    throw e;
} finally {
    sqlSession.close();
}

<insert id="batchInsertUsers" parameterType="java.util.List">
  INSERT INTO users (username, email)
  VALUES
  <foreach collection="list" item="user" separator=",">
    (#{user.username}, #{user.email})
  </foreach>
</insert>

<insert id="insertUserWithSequence">
  <selectKey keyProperty="id" resultType="int" order="BEFORE">
    SELECT user_seq.NEXTVAL FROM dual
  </selectKey>
  INSERT INTO users (id, username, email)
  VALUES (#{id}, #{username}, #{email})
</insert>

<insert id="insertUserWithReturning" parameterType="User">
  INSERT INTO users (username, email)
  VALUES (#{username}, #{email})
  RETURNING id
</insert>

<selectKey keyProperty="id" resultType="int" order="BEFORE">
  SELECT COALESCE(MAX(id), 0) + 1 FROM users -- 简单示例，实际生产环境中应避免这种方式
</selectKey>

log.info("Generated ID for new user: {}", user.getId());