关于MyBatis的小结(第二阶段)

红太狼 2023-06-16 02:26 12阅读 0赞

这里主要是承接上一篇文章[https://blog.csdn.net/kunfd/article/details/103214106][https_blog.csdn.net_kunfd_article_details_103214106]

主要是从下面几个方面进行讲述 1. 是针对于多表查询(1对1，1对多，多对多) 2.查询缓存的理解

### 1.多表查询 ###

1.1.一对一映射

a.在实体类上，需要在实体类A中嵌入实体类B

b.在配置文件中使用<association>

相关文件配置如下

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 1][]

SQL语句

SELECT 
      orders.`id`,
      orders.`user_id`,
      orders.`number`,
      user.`username`,
      user.`sex` 
    FROM
      orders,
      USER 
    WHERE orders.`user_id` = user.`id`

从截图来看，大家主要记住的就是，其实配置文件和对应的实体类本质上是1,1对应的关系的。

1.2.一对多映射

在“一”po类中添加“多”的po类

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 2][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 3][]

注意，这里的extends继承的是 1对1中的部分关系

下面是SQL语句

SELECT 
      orders.`id`,
      orders.`user_id`,
      orders.`number`,
      user.`username`,
      user.`sex`,
      orderdetail.`id` detailId,
      orderdetail.`items_id`,
      orderdetail.`items_num` 
    FROM
      orders,
      USER,
      orderdetail 
    WHERE orders.`user_id` = user.`id` 
      AND orders.`id` = orderdetail.`orders_id`

1.3.多对多

例子中举的是传递以后形成的多对多关系

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 4][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 5][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 6][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 7][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 8][]

### 二.缓存知识 ###

1.一级缓存：指的就是sqlsession，在sqlsession中有一个数据区域，是map结构，这个区域就是一级缓存区域。一级缓存中的key是由sql语句、条件、statement等信息组成一个唯一值。一级缓存中的value，就是查询出的结果对象。默认开启

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 9][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 10][]

2.二级缓存是mapper级别的缓存，多个SqlSession去操作同一个Mapper的sql语句，多个SqlSession可以共用二级缓存，二级缓存是跨SqlSession的。需要手工开启

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 11][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2t1bmZk_size_16_color_FFFFFF_t_70 12][]

对于上图，如果没有红框里面的代码的话，第一次查询会去数据库查询，第二次查询的话只会从缓存中获取已经查询过的数据。不过加了红框中的内容以后，第二次查询也需要进入到数据库查询，因为数据库的数据发生了变化。（***这里要区别于一级缓存，这用的是不同的Mapper对象，而一级缓存使用的是同一个Mapper对象）***

[https_blog.csdn.net_kunfd_article_details_103214106]: https://blog.csdn.net/kunfd/article/details/103214106
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