java每日精进 5.20【MyBatis 联表分页查询】

1. MyBatis XML 实现分页查询

1.1 实现方式

MyBatis XML 是一种传统的 MyBatis 使用方式，通过在 XML 文件中编写 SQL 语句，并结合 Mapper 接口和 Service 层实现分页查询。分页需要手动编写两条 SQL 语句：一条查询分页数据列表，另一条查询总记录数。分页参数（如页码和每页大小）通过 LIMIT 语句手动实现。

1.2 代码解析

以下是文档中提供的 MyBatis XML 分页查询代码的解析：

1.2.1 xml代码

// 获取所有用户（分页，支持用户名模糊查询）@GetMapping("/xmluserpage")public IPage<User> getUsersByPage(@RequestParam(defaultValue = "1") int page,@RequestParam(defaultValue = "10") int size,@RequestParam(required = false) String username) {return userService.getUserPage(page, size, username);}

@Overridepublic IPage<User> getUserPage(int page, int size, String username) {Page<User> userPage = new Page<>(page, size);return userMapper.selectPageCustom(userPage, username);}

// 自定义分页查询（单表）IPage<User> selectPageCustom(Page<User> page, @Param("username") String username);

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.home.mapper.UserMapper"><!-- 自定义分页查询 --><select id="selectPageCustom" resultType="com.home.pojo.User">SELECT id, username, password, email, create_time, update_timeFROM user<where><if test="username != null and username != ''">AND username LIKE CONCAT('%', #{username}, '%')</if></where></select></mapper>

2. MyBatis XML 联表查询

2.1背景代码

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><groupId>org.example</groupId><artifactId>CRUDSystem</artifactId><version>1.0-SNAPSHOT</version><packaging>jar</packaging> <!-- 建议改为jar，Spring Boot默认打包为可执行jar --><name>CRUDSystem</name><url>http://www.example.com</url><properties><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding><maven.compiler.source>17</maven.compiler.source><maven.compiler.target>17</maven.compiler.target><spring-boot.version>2.7.18</spring-boot.version><mybatis-plus.version>3.5.3.1</mybatis-plus.version><mybatis-plus-join.version>1.4.10</mybatis-plus-join.version></properties><dependencies><!-- MyBatis Plus --><dependency><groupId>com.baomidou</groupId><artifactId>mybatis-plus-boot-starter</artifactId><version>${mybatis-plus.version}</version></dependency><!-- MyBatis Plus Join --><dependency><groupId>com.github.yulichang</groupId><artifactId>mybatis-plus-join-boot-starter</artifactId><version>${mybatis-plus-join.version}</version></dependency><dependency><groupId>com.baomidou</groupId><artifactId>mybatis-plus-extension</artifactId><version>${mybatis-plus.version}</version></dependency><!-- MySQL 驱动 --><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>8.0.33</version></dependency><!-- Spring Boot 核心 --><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter</artifactId><version>${spring-boot.version}</version></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId><version>${spring-boot.version}</version></dependency><!-- Lombok --><dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><version>1.18.28</version></dependency><!-- 测试依赖 --><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-test</artifactId><version>${spring-boot.version}</version><scope>test</scope></dependency></dependencies><build><finalName>CRUDSystem</finalName><plugins><!-- Spring Boot 打包插件 --><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId><version>${spring-boot.version}</version><configuration><excludes><exclude><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId></exclude></excludes></configuration></plugin><!-- 编译插件 --><plugin><groupId>org.apache.maven.plugins</groupId><artifactId>maven-compiler-plugin</artifactId><version>3.8.1</version><configuration><source>17</source><target>17</target><release>17</release></configuration></plugin></plugins></build>
</project>

@Data
@TableName("user")
public class User {@TableId(type = IdType.AUTO)private Long id;private String username;private String password;private String email;@TableField(fill = FieldFill.INSERT)private LocalDateTime createTime;@TableField(fill = FieldFill.INSERT_UPDATE)private LocalDateTime updateTime;
}
@Data
@TableName("`order`")
public class Order {@TableId(type = IdType.AUTO)private Long id;private Long userId;private Long productId;private Integer quantity;private BigDecimal totalPrice;private Integer status;@TableField(fill = FieldFill.INSERT)private LocalDateTime createTime;@TableField(fill = FieldFill.INSERT_UPDATE)private LocalDateTime updateTime;
}
//字段平铺
@Data
public class UserOrderDetailDO extends User {private Long orderId;private BigDecimal totalPrice;private Integer orderStatus;
}
//字段内嵌
@Data
public class UserOrderDetail2DO extends User {private Order order;
}

2.2字段平铺

// 字段平铺：获取用户订单详情（分页，订单状态和用户名过滤）@GetMapping("/orders")public IPage<UserOrderDetailDO> getUserOrders(@RequestParam(defaultValue = "1") int page,@RequestParam(defaultValue = "10") int size,@RequestParam Integer status,@RequestParam(required = false) String username) {return userService.getUserOrderPage(page, size, status, username);}@Overridepublic IPage<UserOrderDetailDO> getUserOrderPage(int page, int size, Integer status, String username) {Page<UserOrderDetailDO> userOrderPage = new Page<>(page, size);return userMapper.selectListByStatusAndUsername(userOrderPage, status, username);}
// 字段平铺：联表查询用户和订单（状态为已支付，用户名模糊匹配）default IPage<UserOrderDetailDO> selectListByStatusAndUsername(Page<UserOrderDetailDO> page,@Param("status") Integer status,@Param("username") String username) {return selectJoinPage(page, UserOrderDetailDO.class, new MPJLambdaWrapper<User>() // 改为 selectJoinPage.selectAll(User.class).selectAs(Order::getId, UserOrderDetailDO::getOrderId).selectAs(Order::getTotalPrice, UserOrderDetailDO::getTotalPrice).selectAs(Order::getStatus, UserOrderDetailDO::getOrderStatus).eq(Order::getStatus, status).leftJoin(Order.class, Order::getUserId, User::getId).like(username != null, User::getUsername, username));}

等价于以下SQL

SELECT 
    u.*,
    o.id AS orderId,
    o.total_price AS totalPrice,
    o.status AS orderStatus
FROM 
    user u
LEFT JOIN 
    `order` o ON u.id = o.user_id  -- 注意这里应该是 user_id 而不是 o.id
WHERE 
    o.status = 1 
    AND u.username LIKE '%username%'

查询结果：

{"records": [{"id": 1,"username": "user1","password": "password123","email": "[e-mail]","createTime": "2025-05-20T10:15:33","updateTime": "2025-05-20T10:15:33","orderId": 1,"totalPrice": 999.99,"orderStatus": 1}],"total": 2,"size": 1,"current": 1,"orders": [],"optimizeCountSql": true,"searchCount": true,"maxLimit": null,"countId": null,"pages": 2
}

2.3字段嵌入

// 字段内嵌：获取用户订单详情（分页，订单状态和用户名过滤）@GetMapping("/orders2")public IPage<UserOrderDetail2DO> getUserOrders2(@RequestParam(defaultValue = "1") int page,@RequestParam(defaultValue = "10") int size,@RequestParam Integer status,@RequestParam(required = false) String username) {return userService.getUserOrderPage2(page, size, status, username);}
@Overridepublic IPage<UserOrderDetail2DO> getUserOrderPage2(int page, int size, Integer status, String username) {Page<UserOrderDetail2DO> userOrderPage = new Page<>(page, size);return userMapper.selectList2ByStatusAndUsername(userOrderPage, status, username);}
// 字段内嵌：联表查询用户和订单（状态为已支付，用户名模糊匹配）default IPage<UserOrderDetail2DO> selectList2ByStatusAndUsername(Page<UserOrderDetail2DO> page,@Param("status") Integer status,@Param("username") String username) {return selectJoinPage(page, UserOrderDetail2DO.class, new MPJLambdaWrapper<User>() // 改为 selectJoinPage.selectAll(User.class).selectAssociation(Order.class, UserOrderDetail2DO::getOrder).eq(Order::getStatus, status).leftJoin(Order.class, Order::getUserId, User::getId).like(username != null, User::getUsername, username));}

等价SQL：

SELECT 
    u.*, 
    o.*
FROM 
    t_user u
LEFT JOIN 
    t_order o ON o.user_id = u.id
WHERE 
    o.status = #{status}
    AND (#{username} IS NULL OR u.username LIKE CONCAT('%', #{username}, '%'))
LIMIT #{page.size} OFFSET #{page.offset}

返回值为：

{"records": [{"id": 1,"username": "user1","password": "password123","email": "[e-mail]","createTime": "2025-05-20T10:15:33","updateTime": "2025-05-20T10:15:33","order": {"id": 1,"userId": 1,"productId": 1,"quantity": 1,"totalPrice": 999.99,"status": 1,"createTime": "2025-05-20T10:16:00","updateTime": "2025-05-20T10:16:00"}}],"total": 2,"size": 1,"current": 1,"orders": [],"optimizeCountSql": true,"searchCount": true,"maxLimit": null,"countId": null,"pages": 2
}