mybatis:查询Select

• 单表查询
• 一对一关联查询（非懒加载 or 懒加载）
• 一对多关联查询（懒加载）

表结构介绍

CREATE TABLE users(
    id INT PRIMARY KEY AUTO_INCREMENT,
    username VARCHAR(20),
    PASSWORD VARCHAR(32),
    user_expansion_id INT 
);
CREATE TABLE user_expansion(
    id INT PRIMARY KEY AUTO_INCREMENT,
    address VARCHAR(20)
);
CREATE TABLE user_groups(
    id INT PRIMARY KEY AUTO_INCREMENT,
    group_name VARCHAR(20)
);
CREATE TABLE user_group_relation(
    id INT PRIMARY KEY AUTO_INCREMENT,
    user_id INT,
    group_id INT
);

-- DML 基础数据
INSERT INTO users(username,PASSWORD,user_expansion_id) VALUES ('admin','admin',1);
INSERT INTO user_expansion(address) VALUES ('河北省沧州市XXX');
INSERT INTO user_groups(group_name) VALUES('Administrator');
INSERT INTO user_group_relation(user_id,group_id) VALUES(1,1);
INSERT INTO user_groups(group_name) VALUES('User');
INSERT INTO user_group_relation(user_id,group_id) VALUES(1,2);

1.单表查询（so easy）
根据主键查询用户

<select id="selectById" parameterType="java.lang.Long" resultType="com.xbz.user.entity.User">
        SELECT * FROM users WHERE id = #{id}
select>

2.1一对一关联查询（非懒加载）
根据主键查询用户及其扩展信息

"selectUserAndExpansion" resultType="com.xbz.user.entity.User">
    SELECT
        u.id,u.username,u.password,u.user_expansion_id,
        ue.id AS "expansion.id",
        ue.address AS "expansion.address"
    FROM
        users u,user_expansion ue
    WHERE
        u.user_expansion_id=ue.id
        AND u.id=#{id}

2.2一对一关联查询（懒加载）

注意：使用懒加载必须需要在mybatis的configure文件配置：
lazyLoadingEnabled
aggressiveLazyLoading

<setting name="mapUnderscoreToCamelCase" value="true"/>

<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>

<select id="selectUserAndLazyExpansion" resultMap="userMap">
    SELECT
        id,username,password,
        user_expansion_id AS "expansionId"
    FROM
        users
    WHERE
        id=#{id}
select>
<select id="selectExpansionById" resultType="com.xbz.user.entity.UserExpansion">
    SELECT
        id,address
    FROM
        user_expansion
    WHERE
        id=#{id}
select>
<resultMap id="userMap" type="com.xbz.user.entity.User">
        <id column="id" property="id"/>
        <result column="username" property="username"/>
        <result column="password" property="password"/>
    
    <association 
        property="expansion"  
        column="expansionId" 
        select="selectExpansionById">
    association>
resultMap>

3.多对多关联查询（懒加载）

<select id="selectUserAndGroups" resultMap="userMap">
    SELECT
        id,username,password,user_expansion_id 
    FROM
        users
    WHERE
        id=#{id}
select>
<select id="selectGroupsByUserId" resultType="com.xbz.user.entity.UserGroup">
    SELECT
        g.id,g.group_name
    FROM
        user_groups g,user_group_relation r
    WHERE
        g.id=r.group_id
        AND r.user_id=#{uid}
select>
<resultMap id="userMap" type="com.xbz.user.entity.User">
    <id column="id" property="id"/>
    <result column="username" property="username"/>
    <result column="password" property="password"/>
    
    <association 
        property="expansion"  
        column="expansionId" 
        select="selectExpansionById">
    association>
    
    
    <collection 
        property="groups" 
        column="id"
        ofType="com.xbz.user.entity.UserGroup"
        select="selectGroupsByUserId">
        <id column="id" property="id"/>
        <result column="group_name" property="groupName" />
    collection>
resultMap>

完整配置
mapper Class

package com.xbz.user.mapper;

import java.io.Serializable;

import com.xbz.user.entity.User;

public interface UserMapper {
    public User selectById(Serializable id);
    public void save(User user);
    public void deleteById(Serializable id);
    public void update(User user);
    /**
     * 根据id查询用户并获取用户拓展信息
     * 
测试mybatis的一对一关联查询（非懒加载）
     * @param id 
     * @return
     */
    public User selectUserAndExpansion(Serializable id);
    /**
     * 根据id查询用户并懒加载获取用户拓展信息
     * 
测试mybatis的一对一关联查询（懒加载）
     * @param id
     * @return
     */
    public User selectUserAndLazyExpansion(Serializable id);
    /**
     * 根据id查询用户并获取用户所在用户组信息
     * 
测试mybatis的一对多关联查询（懒加载）
     * @param id
     * @return
     */
    public User selectUserAndGroups(Serializable id);

}

相关实体类
getter、setter省略

package com.xbz.user.entity;
import java.io.Serializable;
import java.util.Set;

public class User implements Serializable{
    private static final long serialVersionUID = 1L;
    private Long id;
    private String username;
    private String password;
    private UserExpansion expansion;
    private Set groups;
}

public class UserExpansion implements Serializable {
    private static final long serialVersionUID = 1L;
    private Long id;
    private String address;
}

public class UserGroup implements Serializable{
    private static final long serialVersionUID = 1L;
    private Long id;
    private String groupName;
}

mapper.xml

<mapper namespace="com.xbz.user.mapper.UserMapper">
    <select id="selectById" parameterType="java.lang.Long" resultType="com.xbz.user.entity.User">
        SELECT * FROM users WHERE id = #{id}
    select>
    <select id="selectUserAndExpansion" resultType="com.xbz.user.entity.User">
        SELECT
            u.id,u.username,u.password,u.user_expansion_id,
            ue.id AS "expansion.id",
            ue.address AS "expansion.address"
        FROM
            users u,user_expansion ue
        WHERE
            u.user_expansion_id=ue.id
            AND u.id=#{id}
    select>
    <select id="selectUserAndLazyExpansion" resultMap="userMap">
        SELECT
            id,username,password,user_expansion_id AS "expansionId"
        FROM
            users
        WHERE
            id=#{id}
    select>
    <select id="selectExpansionById" resultType="com.xbz.user.entity.UserExpansion">
        SELECT
            id,address
        FROM
            user_expansion
        WHERE
            id=#{id}
    select>
    <select id="selectUserAndGroups" resultMap="userMap">
        SELECT
            id,username,password,user_expansion_id 
        FROM
            users
        WHERE
            id=#{id}
    select>
    <select id="selectGroupsByUserId" resultType="com.xbz.user.entity.UserGroup">
        SELECT
            g.id,g.group_name
        FROM
            user_groups g,user_group_relation r
        WHERE
            g.id=r.group_id
            AND r.user_id=#{uid}
    select>
    <resultMap id="userMap" type="com.xbz.user.entity.User">
        <id column="id" property="id"/>
        <result column="username" property="username"/>
        <result column="password" property="password"/>
        
        <association 
            property="expansion"  
            column="expansionId" 
            select="selectExpansionById">
        association>
        
        
        <collection 
            property="groups" 
            column="id"
            ofType="com.xbz.user.entity.UserGroup"
            select="selectGroupsByUserId">
            <id column="id" property="id"/>
            <result column="group_name" property="groupName" />
        collection>
    resultMap>
mapper>

测试类

package com.xbz.test.mapper;

import java.util.Arrays;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

import com.xbz.user.entity.User;
import com.xbz.user.entity.UserExpansion;
import com.xbz.user.mapper.UserMapper;

/**
 * Spring测试配置使用如下注解
 */
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations="classpath:spring/*.xml")
public class UserMapperTest {
    @Autowired
    private UserMapper userMapper;
    @Test
    public void testSelectById(){
        User user = userMapper.selectById(1L);
        System.out.println(user);
    }
    @Test
    public void testSelectUserAndExpansion(){
        User user = userMapper.selectUserAndExpansion(1L);
        System.out.println("-----------------------------------------------------------");
        user.getExpansion();
        System.out.println(user);
    }
    @Test
    public void testSelectUserAndLazyExpansion(){
        User user = userMapper.selectUserAndLazyExpansion(1L);
        System.out.println("-----------------------------------------------------------");
        UserExpansion expansion = user.getExpansion();
        System.out.println(expansion);
    }

    @Test
    public void testSelectUserAndGroups(){
        User user = userMapper.selectUserAndGroups(1L);
        System.out.println("-----------------------------------------------------------");
        System.out.println(Arrays.toString(user.getGroups().toArray()));    
    }
}