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# Mybatis初步复习
## 1.SpringBoot整合Mybatis
## 1.1第一步添加依赖
```<!-- mybatis启动器--> <dependency> <groupId>org.mybatis.spring.boot</groupId> <artifactId>mybatis-spring-boot-starter</artifactId> <version>3.0.5</version> </dependency>
```
### 1.2进行配置
```#配置Mybatis
mybatis: configuration: #在映射为java对象，将表中的下划线命名自动转换成驼峰式命名 map-underscore-to-camel-case: true #日志前缀 可选 log-prefix: mybatis. #日志实现类 可选 log-impl: org.apache.ibatis.logging.commons.JakartaCommonsLoggingImpl #动态sql文件存储位置 mapper-locations: classpath:/mapper/**/*.xml
#配置日志显示sql
logging: level: #指定日志前缀 mybatis: debug
```
### 1.3 在Dao层编写的mapper接口，添加@Mapper注解
### 1.4 编写动态sql文件（xx.xml文件）
### 1.5 安装mybatisx插件
安装之后在mapper和动态SQL文件对应代码左边有一个小鸟图标，方便直接跳转
## 2.自动映射
当开启驼峰命名自动映射
```yamlmybatis: configuration: map-underscore-to-camel-case: true```
## 3.手动映射
当自动映射无法满足需求时，可以使用 `<resultMap>` 进行精确的手动映射。
```xml<resultMap id="唯一标识" type="要映射的Java类型"> <!-- 主键映射 --> <id property="Java属性名" column="数据库列名"/> <!-- 普通字段映射 --> <result property="Java属性名" column="数据库列名"/> <!-- 关联关系映射 --> <association property="关联对象属性" javaType="关联对象类型"/> <collection property="集合属性" ofType="集合元素类型"/></resultMap>```
举例：
### 3.1基础字段映射
```java实体类public class User { private Long userId; // 与数据库字段名不同 private String userName; // 需要手动映射 private String email; private Integer status; private LocalDateTime createTime; private LocalDateTime updateTime; // 构造函数、getter、setter...}```
```sql数据库表结构：CREATE TABLE t_user ( id BIGINT PRIMARY KEY, name VARCHAR(50), email VARCHAR(100), status TINYINT, created_at DATETIME, updated_at DATETIME);
```
```xml<!-- 基础手动映射示例 --><resultMap id="BaseUserMap" type="User"> <!-- 主键字段，使用 id 标签 --> <id property="userId" column="id"/> <!-- 普通字段，使用 result 标签 --> <result property="userName" column="name"/> <result property="email" column="email"/> <result property="status" column="status"/> <result property="createTime" column="created_at"/> <result property="updateTime" column="updated_at"/></resultMap>
<!-- 使用 resultMap --><select id="selectUserById" resultMap="BaseUserMap"> SELECT id, name, email, status, created_at, updated_at FROM t_user WHERE id = #{id}</select>```
说明：
### 3.2一对一关联映射
#### 3.2.1单次查询嵌套结果映射
举例：
```java实体类public class User { private Long userId; private String userName; private UserProfile userProfile; // 一对一关联 // getter/setter...}
public class UserProfile { private Long profileId; private Long userId; private String realName; private Integer age; private String address; private String phone; // getter/setter...}```
```xml手动映射配置：<!-- 用户和用户详情的一对一映射 --><resultMap id="UserWithProfileMap" type="User"> <id property="userId" column="id"/> <result property="userName" column="name"/> <!-- association: 一对一关联映射 --> <association property="userProfile" javaType="UserProfile"> <id property="profileId" column="profile_id"/> <result property="userId" column="id"/> <!-- 注意：这里复用外层查询的id --> <result property="realName" column="real_name"/> <result property="age" column="age"/> <result property="address" column="address"/> <result property="phone" column="phone"/> </association></resultMap>
<!-- 关联查询SQL --><select id="selectUserWithProfile" resultMap="UserWithProfileMap"> SELECT u.id, u.name, up.id as profile_id, up.real_name, up.age, up.address, up.phone FROM t_user u LEFT JOIN t_user_profile up ON u.id = up.user_id WHERE u.id = #{userId}</select>```
#### 3.2.2 多次查询嵌套查询映射
```xml<!-- 主结果映射 --><resultMap id="UserWithProfileNestedMap" type="User"> <id property="userId" column="id"/> <result property="userName" column="name"/> <!-- 嵌套查询：通过 select 属性引用另一个查询 --> <association property="userProfile" column="id" select="selectUserProfileByUserId"/></resultMap>
<!-- 主查询 --><select id="selectUserWithProfileNested" resultMap="UserWithProfileNestedMap"> SELECT id, name FROM t_user WHERE id = #{userId}</select>
<!-- 嵌套查询 --><select id="selectUserProfileByUserId" resultType="UserProfile"> SELECT id as profileId, user_id as userId, real_name as realName, age, address, phone FROM t_user_profile` WHERE user_id = #{userId}</select>```
### 3.3一对多集合映射
```java实体类：public class User { private Long userId; private String userName; private List<Order> orders; // 一对多关联 // getter/setter...}
public class Order { private Long orderId; private Long userId; private String orderNumber; private BigDecimal amount; private LocalDateTime orderTime; // getter/setter...}```
```xml<!-- 用户和订单的一对多映射 --><resultMap id="UserWithOrdersMap" type="User"> <id property="userId" column="id"/> <result property="userName" column="name"/> <!-- collection: 一对多关联映射 --> <collection property="orders" ofType="Order"> <id property="orderId" column="order_id"/> <result property="userId" column="id"/> <!-- 注意：这里复用外层查询的id --> <result property="orderNumber" column="order_number"/> <result property="amount" column="amount"/> <result property="orderTime" column="order_time"/> </collection></resultMap>
<!-- 关联查询SQL --><select id="selectUserWithOrders" resultMap="UserWithOrdersMap"> SELECT u.id, u.name, o.id as order_id, o.order_number, o.amount, o.order_time FROM t_user u LEFT JOIN t_order o ON u.id = o.user_id WHERE u.id = #{userId}</select>```
### 3.4复杂的多层嵌套
```javapublic class User { private Long userId; private String userName; private List<Order> orders;}
public class Order { private Long orderId; private String orderNumber; private List<OrderItem> orderItems;}
public class OrderItem { private Long itemId; private Long productId; private Integer quantity; private Product product; // 关联商品}
public class Product { private Long productId; private String productName; private BigDecimal price;}```
```xml<!-- 完整的多层嵌套映射 --><resultMap id="CompleteUserMap" type="User"> <id property="userId" column="user_id"/> <result property="userName" column="user_name"/> <!-- 第一层嵌套：订单集合 --> <collection property="orders" ofType="Order" resultMap="OrderWithItemsMap"/></resultMap>
<resultMap id="OrderWithItemsMap" type="Order"> <id property="orderId" column="order_id"/> <result property="orderNumber" column="order_number"/> <!-- 第二层嵌套：订单项集合 --> <collection property="orderItems" ofType="OrderItem" resultMap="OrderItemMap"/></resultMap>
<resultMap id="OrderItemMap" type="OrderItem"> <id property="itemId" column="item_id"/> <result property="quantity" column="quantity"/> <!-- 第三层嵌套：商品信息 --> <association property="product" javaType="Product"> <id property="productId" column="product_id"/> <result property="productName" column="product_name"/> <result property="price" column="price"/> </association></resultMap>
<!-- 复杂查询SQL --><select id="selectUserWithOrderDetails" resultMap="CompleteUserMap"> SELECT u.id as user_id, u.name as user_name, o.id as order_id, o.order_number, oi.id as item_id, oi.quantity, p.id as product_id, p.name as product_name, p.price FROM t_user u LEFT JOIN t_order o ON u.id = o.user_id LEFT JOIN t_order_item oi ON o.id = oi.order_id LEFT JOIN t_product p ON oi.product_id = p.id WHERE u.id = #{userId} ORDER BY o.order_time DESC, oi.id</select>```
### 3.5.注意
1.可以 自动映射与手动映射结合
```xml<!-- 混合映射：autoMapping="true" 开启自动映射 --><resultMap id="HybridUserMap" type="User" autoMapping="true"> <!-- 只手动映射名称不匹配的字段 --> <id property="userId" column="id"/> <result property="userName" column="name"/> <result property="createTime" column="created_at"/> <!-- 关联对象也可以使用自动映射 --> <association property="userProfile" javaType="UserProfile" autoMapping="true"> <id property="profileId" column="profile_id"/> <!-- 只覆盖需要特殊处理的字段 --> <result property="realName" column="real_name"/> </association></resultMap>```
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