Mybatis之多表查询

  • 开发
  • 37

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

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 mapper
        PUBLIC "-//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;
    @Before
    public void creatFactory(){
        InputStream input = null;
        try {
            input = Resources.getResourceAsStream("SqlMapConfig.xml");
        } catch (IOException e) {
            e.printStackTrace();
        }
        ssf = new SqlSessionFactoryBuilder().build(input);
    }

@Test
    public 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：

 SELECT
  orders.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,items


WHERE orders.student_id = student.id
  AND orders.id = orderdetail.orders_id
  AND 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 mapper
        PUBLIC "-//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">
        SELECT
		  orders.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,items


		WHERE orders.student_id = student.id
		  AND orders.id = orderdetail.orders_id
		  AND 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;
    @Before
    public void creatFactory(){
        InputStream input = null;
        try {
            input = Resources.getResourceAsStream("SqlMapConfig.xml");
        } catch (IOException e) {
            e.printStackTrace();
        }
        ssf = new SqlSessionFactoryBuilder().build(input);
    }

    @Test
    public 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());
                }
            }
        }
    }

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