Mybatis之多表查询

目录

一、简介

1、使用嵌套查询:

2、使用多个 SQL 语句：

3、使用关联查询：

4、使用自定义映射查询：

二、业务场景

三、示例

1、一对一查询

2、一对多查询

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一、简介

MyBatis 是一个优秀的持久层框架，它提供了强大的支持来执行数据库操作，包括多表查询。多表查询是指从多个数据库表中检索数据的过程，这在实际的应用中非常常见。MyBatis 提供了多种方法来执行多表查询，以下是一些常见的技术和方法：

1、使用嵌套查询:

这是最基本的多表查询方法，通过在 SQL 语句中嵌套子查询来联合多个表的数据。例如，你可以在 SELECT 语句中使用子查询来从一个表中获取数据，然后再将其用于主查询中。这种方法在某些简单情况下是有效的，但对于复杂的多表查询可能会变得冗长和难以维护。

2、使用多个 SQL 语句：

在 MyBatis 中，你可以编写多个独立的 SQL 语句，每个语句都从一个表中检索数据，然后在 Java 代码中将这些数据组合起来。这通常需要在 Java 代码中手动执行每个查询并进行数据处理，但对于一些复杂的多表查询情况可能更加灵活。

3、使用关联查询：

 MyBatis 支持使用关联查询来执行多表查询，特别是在映射文件中配置了表之间的关联关系。通过在 Mapper XML 文件中配置 association 或 collection 来表示表之间的关联关系，MyBatis 可以自动根据关系从多个表中检索数据并构造结果对象。

4、使用自定义映射查询：

有时候，多表查询的结果可能不适合于一个实体类，这时你可以使用自定义映射查询来将结果映射到一个 Map 或者其他自定义的数据结构中，以适应查询的需要。

二、业务场景

模拟的业务场景为订单与用户的关系，可以是一对一、一对多。

比如，一个用户有多个订单，一个订单只有一个用户。

还有就是用户与角色的关系，多个用户拥有多个角色，一个角色可以被多个用户拥有，一个用户可以拥有多个角色。

三、示例

1、一对一查询

案例：查询所有订单信息，关联查询下单用户信息。 注意：因为一个订单信息只会是一个人下的订单，所以从查询订单信息出发关联查询用户信息为一对一查询。如果从用户信息出发查询用户下的订单信息则为一对多查询，因为一个用户可以下多个订单。

一对一查询的需求：查询一个订单，与此同时查询出该订单所属的用户

数据库：

对应的SQL语句：

select o.id,o.ordername,o.ordercount,s.studentname,s.address from orders o,student s where o.student_id = s.id

用户表student

public class Student {private int id;private String studentName;private String gender;private String address;private String email;private String remark;public Student() {}public Student( String studentName, String gender, String address, String email, String remark) {this.studentName = studentName;this.gender = gender;this.address = address;this.email = email;this.remark = remark;}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getStudentName() {return studentName;}public void setStudentName(String studentName) {this.studentName = studentName;}public String getGender() {return gender;}public void setGender(String gender) {this.gender = gender;}public String getAddress() {return address;}public void setAddress(String address) {this.address = address;}public String getEmail() {return email;}public void setEmail(String email) {this.email = email;}public String getRemark() {return remark;}public void setRemark(String remark) {this.remark = remark;}}
}

订单表order

public class Order {private int id;private String ordername;private int ordercount;private Student student;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getOrdername() {return ordername;}public void setOrdername(String ordername) {this.ordername = ordername;}public int getOrdercount() {return ordercount;}public void setOrdercount(int ordercount) {this.ordercount = ordercount;}public Student getStudent() {return student;}public void setStudent(Student student) {this.student = student;}
}

StudentMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.xinhua.mapper.StudentMapper"><!--查询出每个用户的信息和对应的所有订单--><resultMap id="studentOrder" type="Student"><id column="id" property="id"></id><result column="studentName" property="studentName"></result><result column="gender" property="gender"></result><result column="address" property="address"></result><collection property="listOrder" ofType="com.xinhua.domain.Order"><id column="order_id" property="id"></id><result column="ordername" property="ordername"></result><result column="ordercount" property="ordercount"></result></collection></resultMap><select id="findStudentOrderAll" resultMap="studentOrder">SELECT s.id,s.studentname,s.gender,s.address,o.id order_id,o.ordername,o.ordercount FROM student s LEFT JOIN Orders o ON s.id=o.student_id</select>
</mapper>

mapper接口类

public interface StudentMapper {public List<Student> findStudentOrderAll();
}

测试类：

public class TestDemo {SqlSessionFactory ssf = null;@Beforepublic void creatFactory(){InputStream input = null;try {input = Resources.getResourceAsStream("SqlMapConfig.xml");} catch (IOException e) {e.printStackTrace();}ssf = new SqlSessionFactoryBuilder().build(input);}@Testpublic void testMapper10() {SqlSession sqlSession = ssf.openSession();StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);List<Student> students = mapper.findStudentOrderAll();for (Student student : students){System.out.println(student.getId()+","+student.getStudentName()+","+student.getGender()+","+student.getAddress());List<Order> listOrder = student.getListOrder();for (Order order : listOrder){System.out.println("   "+order.getOrdername()+","+order.getOrdercount());}}}

2、一对多查询

用户表和订单表的关系为，一个用户有多个订单，一个订单只从属于一个用户

一对多查询的需求：查询一个用户，与此同时查询出该用户具有的订单

主查询表：订单表

关联查询表：用户表、订单明细表、商品表

订单表

用户表

商品表

订单明细表 

对应的SQL语句：

 SELECTorders.id o_id,orders.ordername,student.studentname,student.gender,orderdetail.id od_id,orderdetail.items_count,orderdetail.items_all_price ,items.id it_id,items.items_name,items.items_price
FROM ORDERs,student,orderdetail,itemsWHERE orders.student_id = student.idAND orders.id = orderdetail.orders_idAND orderdetail.items_id = items.id

用户表

相比之前一对一，变化在student类多了一个List<Order> orderList

public class Student {private int id;private String studentName;private String gender;private String address;private String email;private String remark;private List<Order> listOrder;public Student() {}public Student( String studentName, String gender, String address, String email, String remark) {this.studentName = studentName;this.gender = gender;this.address = address;this.email = email;this.remark = remark;}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getStudentName() {return studentName;}public void setStudentName(String studentName) {this.studentName = studentName;}public String getGender() {return gender;}public void setGender(String gender) {this.gender = gender;}public String getAddress() {return address;}public void setAddress(String address) {this.address = address;}public String getEmail() {return email;}public void setEmail(String email) {this.email = email;}public String getRemark() {return remark;}public void setRemark(String remark) {this.remark = remark;}public List<Order> getListOrder() {return listOrder;}public void setListOrder(List<Order> listOrder) {this.listOrder = listOrder;}
}

Order：

public class Order {private int id;private String ordername;private int ordercount;private Student student;private List<OrderDetail> orderDetailList;public List<OrderDetail> getOrderDetailList() {return orderDetailList;}public void setOrderDetailList(List<OrderDetail> orderDetailList) {this.orderDetailList = orderDetailList;}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getOrdername() {return ordername;}public void setOrdername(String ordername) {this.ordername = ordername;}public int getOrdercount() {return ordercount;}public void setOrdercount(int ordercount) {this.ordercount = ordercount;}public Student getStudent() {return student;}public void setStudent(Student student) {this.student = student;}
}

商品表items

    private int id;private String items_name;private int items_price;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getItems_name() {return items_name;}public void setItems_name(String items_name) {this.items_name = items_name;}public int getItems_price() {return items_price;}public void setItems_price(int items_price) {this.items_price = items_price;}
}

订单明细表 

public class OrderDetail {private int id;private int items_count;private int items_all_price;private Order order;private List<Items> listItems;public List<Items> getListItems() {return listItems;}public void setListItems(List<Items> listItems) {this.listItems = listItems;}public Order getOrder() {return order;}public void setOrder(Order order) {this.order = order;}public int getId() {return id;}public void setId(int id) {this.id = id;}public int getItems_count() {return items_count;}public void setItems_count(int items_count) {this.items_count = items_count;}public int getItems_all_price() {return items_all_price;}public void setItems_all_price(int items_all_price) {this.items_all_price = items_all_price;}
}

OrderMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.xinhua.mapper.OrderMapper">
<resultMap id="orderStudentDetailItems" type="com.xinhua.domain.Order"><id column="o_id" property="id"></id><result column="ordername" property="ordername"></result><association property="student" javaType="Student"><result column="studentName" property="studentName"></result><result column="gender" property="gender"></result></association><!-- 一对多关联映射 --><collection property="orderDetailList" ofType="com.xinhua.domain.OrderDetail"><id column="od_id" property="id"></id><result column="items_count" property="items_count"></result><result column="items_all_price" property="items_all_price"></result><collection property="listItems" ofType="com.xinhua.domain.Items"><id column="it_id" property="id"></id><result column="items_name" property="items_name"></result><result column="items_price" property="items_price"></result></collection></collection></resultMap><select id="findOrderStudentDetailItems" resultMap="orderStudentDetailItems">SELECTorders.id o_id,orders.ordername,student.studentname,student.gender,orderdetail.id od_id,orderdetail.items_count,orderdetail.items_all_price ,items.id it_id,items.items_name,items.items_priceFROM ORDERs,student,orderdetail,itemsWHERE orders.student_id = student.idAND orders.id = orderdetail.orders_idAND orderdetail.items_id = items.id</select>
</mapper>

mapper接口类

public interface OrderMapper {public List<Order> findOrderStudentDetailItems();
}

测试类：

public class TestDemo {SqlSessionFactory ssf = null;@Beforepublic void creatFactory(){InputStream input = null;try {input = Resources.getResourceAsStream("SqlMapConfig.xml");} catch (IOException e) {e.printStackTrace();}ssf = new SqlSessionFactoryBuilder().build(input);}@Testpublic void testMapper11() {SqlSession sqlSession = ssf.openSession();OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);List<Order> orderList = mapper.findOrderStudentDetailItems();for (Order order : orderList){System.out.println(order.getId()+","+order.getOrdername()+","+order.getOrdercount()+","+order.getStudent().getStudentName());List<OrderDetail> orderDetailList = order.getOrderDetailList();for (OrderDetail orderDetail : orderDetailList){System.out.println("   "+orderDetail.getId()+","+orderDetail.getItems_count()+","+orderDetail.getItems_all_price());List<Items> list = orderDetail.getListItems();for (Items listItems : list){System.out.println("        "+listItems.getId()+","+listItems.getItems_name()+","+listItems.getItems_price());}}}}