Mybatis多表查询

多对一 ：

　SqlMapConfig.xml

<!--typeAliases标签用于起别名，这样package指定包中的实体类的类名可以在Mapper.xml的resultType中直接使用，无需写全限定类名-->

<typeAliases>
    <package name="com.company.domain"/>
</typeAliases>

<environments default="mysql">
...

</environments>

   实体类

　　public class Account {

    private Integer id;
    private Integer uid;
    private Double money;
    /**
     * 多对一实体属性
     */
    private User user;
　　setter和getter省略。。。
}

Mapper映射文件

<resultMap id="accountMap" type="Account">
    <id property="id" column="aid"></id>
    <result property="uid" column="uid"></result>
    <result property="money" column="money"></result>　　

　　<!--配置多对一实体属性，javaType属性如果未在配置文件中取别名，则需写全限定类名-->

    <association property="user" javaType="user">
        <id property="id" column="id"></id>
        <result property="username" column="username"></result>
        <result property="birthday" column="birthday"></result>
        <result property="sex" column="sex"></result>
        <result property="address" column="address"></result>
    </association>
</resultMap>
<select id="findAll" resultMap="accountMap">

SELECT
a.id aid,
a.uid,
a.money,
u.*
FROM
USER u
RIGHT JOIN account a ON u.id = a.uid;

</select>

一对多 ：
　实体类

public class User {
    private Integer id;
    private String username;
    private Date birthday;
    private Character sex;
    private String address;

    private List<Account> accounts;

　　 setter和getter省略。。。
}

　　
　Mapper映射文件
　　

<resultMap id="userMap" type="user">
    <id property="id" column="id"></id>
    <result property="username" column="username"></result>
    <result property="birthday" column="birthday"></result>
    <result property="sex" column="sex"></result>
    <result property="address" column="address"></result>
    <!--配置一对多属性-->
    <collection property="accounts" ofType="Account">
        <id property="id" column="aid"></id>
        <result property="uid" column="uid"></result>
        <result property="money" column="money"></result>
    </collection>
</resultMap>
<select id="findAll" resultMap="userMap">
    SELECT
        u.*,
        a.id aid,
        a.uid,
        a.money
    FROM
        USER u
    LEFT JOIN account a ON u.id = a.uid;
</select>

多对多 ：

　　实体类　　

public class User {
    private Integer id;
    private String username;
    private Date birthday;
    private Character sex;
    private String address;

    private List<Account> accounts;

    private List<Role> roles;
}

　　Mapper映射文件
　　

<resultMap id="userMap" type="user">
    <id property="id" column="id"></id>
    <result property="username" column="username"></result>
    <result property="birthday" column="birthday"></result>
    <result property="sex" column="sex"></result>
    <result property="address" column="address"></result>
    <!--配置一对多属性，在role的dao中反向也一对多，即完成了多对多-->
    <collection property="accounts" ofType="Account">
        <id property="id" column="aid"></id>
        <result property="uid" column="uid"></result>
        <result property="money" column="money"></result>
    </collection>
</resultMap>

<select id="findRoleAll" resultMap="roleMap">
    SELECT
        u.*,
        r.id rid,
        r.role_desc,
        r.role_name
    FROM
        USER u
    LEFT JOIN user_role ur ON u.id = ur.uid
    LEFT JOIN role r ON ur.rid = r.id;
</select>