7、MyBatis分页

思考：为什么要分页？

• 减少数据的处理量

-- 语法：
select * from user limit startIndex,pageSize;
select * from user limit 3; -- [0,n]
select * from user limit 3,-1; -- mysql历史bug【参考wiki：https://blog.csdn.net/wanghaicheng0421/article/details/77498723 bug地址：https://bugs.mysql.com/bug.php?id=2037】

使用Mybatis实现分页，核心SQL

1.接口

//分页
List getUserByLimit (Map map);

2.Mapper.xml

    select * from mybatis.user limit #{startIndex},#{pageSize}

3.测试

@Test
public void getUserByLimit() {
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    UserMapper mapper = sqlSession.getMapper(UserMapper.class);
    HashMap map = new HashMap();
    map.put("startIndex", 0);
    map.put("pageSize", 2);
    List userList = mapper.getUserByLimit(map);
    for (User user : userList) {
        System.out.println(user);
    }
    sqlSession.close();
}

不再使用SQL实现分页

1.接口

//分页2
List getUserByRowBounds ();

2.mapper.xml

    select * from mybatis.user

3.测试

@Test
public void getUserByRowBounds() {
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    //RowBounds实现
    RowBounds rowBounds = new RowBounds(1, 2);
    //通过java代码层面实现分页
    List userList = sqlSession.selectList("com.gongyi.dao.UserMapper.getUserByRowBounds", null, rowBounds);
    for (User user : userList) {
        System.out.println(user);
    }
    sqlSession.close();
}

了解即可，万一以后公司的架构师说要使用，你要知道它是什么东西

核心思想：所有分页底层都是用的sql层面的limit，只不过做了封装而已

mybatis分页插件官网