Mybatis 支持关联查询吗？

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回答

支持。Mybatis 通过 association、collection 标签来实现一对一和一对多的关联查询。

一对一关联查询

一对一关联查询支持两种方式：嵌套查询和结果集嵌套映射。核心是通过 association 标签支持。

public class User {
    private Integer id;
    private String name;
    private Address address;
    // getters and setters
}

public class Address {
    private Integer id;
    private String city;
    private String province;
    // getters and setters
}

• 嵌套查询（Nested Queries）

<mapper namespace="com.damingge.mapper.UserMapper">
    <resultMap id="userMap" type="com.damingge.entity.User">
        <id property="id" column="id" />
        <result property="username" column="username" />
        <association property="address" resultMap="addressMap" />
    </resultMap>

    <resultMap id="addressMap" type="com.damingge.entity.Address">
        <id property="id" column="address_id" />
        <result property="city" column="city" />
        <result property="country" column="country" />
    </resultMap>

    <select id="selectUserById" resultMap="userResultMap">
        SELECT u.id, u.name, a.id as address_id, a.city, a.province
        FROM t_user u
        LEFT JOIN t_address a ON u.address_id = a.id
        WHERE u.id = #{id}
    </select>
</mapper>

• 结果集嵌套映射（Result Map With Association）

<mapper namespace="com.damingge.mapper.UserMapper">
    <resultMap id="userResultMap" type="com.damingge.entity.User">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <association property="address" column="address_id" javaType="com.damingge.entity.Address">
            <id property="id" column="address_id"/>
            <result property="city" column="city"/>
            <result property="province" column="province"/>
        </association>
    </resultMap>

    <select id="selectUserById" resultMap="userResultMap">
        SELECT u.id, u.name, a.id as address_id, a.city, a.province
        FROM t_user u
        LEFT JOIN t_address a ON u.address_id = a.id
        WHERE u.id = #{id}
    </select>
</mapper>

一对多关联查询

一对多关联查询也支持两种方式：嵌套查询和结果集嵌套映射。核心是通过 collection 标签支持。

public class Department {
    private Integer id;
    private String name;
    private List<Employee> employees;
    // getters and setters
}

public class Employee {
    private Integer id;
    private String name;
    private Integer departmentId;
    // getters and setters
}

• 嵌套查询（Nested Queries）

<mapper namespace="com.damingge.mapper.DepartmentMapper">
    <resultMap id="departmentResultMap" type="com.damingge.entity.Department">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <collection property="employees" ofType="com.damingge.entity.Employee" select="selectOrdersByDepartmentId"/>
    </resultMap>

    <resultMap id="employeeResultMap" type="com.damingge.entity.Employee">
          <id property="id" column="employee_id"/>
          <result property="name" column="employee_name"/>
          <result property="departmentId" column="department_id"/>
    </resultMap>
  

    <select id="selectDepartmentById" resultMap="departmentResultMap">
        SELECT d.id, d.name, d.employee_id
        FROM t_department d
        WHERE d.id = #{id}
    </select>
    <select id="selectEmployeesByDepartmentId" resultMap="employeeResultMap">
        SELECT e.id, e.name, e.department_id
        FROM t_employee e
        WHERE e.department_id = #{departmentId}
    </select>
</mapper>

• 集合嵌套结果映射

<mapper namespace="com.damingge.mapper.DepartmentMapper">
    <resultMap id="departmentResultMap" type="com.damingge.entity.Department">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <collection property="employees" ofType="com.damingge.entity.Employee">
            <id property="id" column="employee_id"/>
            <result property="name" column="employee_name"/>
            <result property="departmentId" column="department_id"/>
        </collection>
    </resultMap>

    <select id="selectDepartmentById" resultMap="departmentResultMap">
        SELECT d.id, d.name, e.id as employee_id, e.name as employee_name, e.department_id
        FROM t_department d
        LEFT JOIN t_employee e ON d.id = e.department_id
        WHERE d.id = #{id}
    </select>
</mapper>

扩展

嵌套查询 VS 嵌套结果

• 嵌套查询：嵌套查询是指在主查询中，对于每一条记录，再单独执行一次子查询来获取关联数据。这种方式通过多次数据库查询实现关联数据的获取，存在 N+1 问题，适用于数据量较小的场景。
• 嵌套结果：嵌套结果是指一次查询就获取所有关联数据，然后在 MyBatis 中通过 ResultMap 进行结果集的解析和映射。单次查询获取所有关联数据，适用于数据量较大的场景，但 SQL 语句复杂度高。