MyBatis映射文件的resultMap如何做表级联查询

MyBatis的核心是其映射文件,SqlMap文件,里面配置了项目中用到了什么SQL语句,和数据库相关的逻辑都在这个映射文件里.顾名思义,映射文件就是对Java对象和SQL的映射.这里简单介绍一下映射文件中resultMap的用法:resultMap – 它描述如何将结果集映射到Java对象.
resultMap属性：type为java实体类；id为此resultMap的标识:<resultMap id="BaseResultMap" type="com.test.mybatis.vo.MybatisOrder" >
resultMap的子元素：
id – 一般对应到数据库中该行的ID,设置此项可以提高Mybatis性能.
result – 映射到JavaBean 的某个"简单类型"属性,String,int等.
association – 映射到JavaBean 的某个"复杂类型"属性,其他JavaBean类.
collection –复杂类型集合,a collection of complex types
比如现在有一个Order表,Customer表和OrderItem表,它们之间的关系为:一个Order关联到一个Customer(单向关联),一个Order有多个OrderItem(双向关联).

对应的Java对象文件为,

public class MybatisOrder {....private Mybatiscustomer customer;private List<MybatisOrderItem> itemList;....
public class MybatisOrderItem {private MybatisOrder order;...

 对应的mapper文件为:

MybatisOrderMapper.xml:

<resultMap id="AssociationSelectMap" type="com.test.mybatis.vo.MybatisOrder" ><id column="ORDERID" property="orderid" jdbcType="DECIMAL" /><result column="ORDERTYPE" property="ordertype" jdbcType="VARCHAR" /><result column="ORDERDATE" property="orderdate" jdbcType="DATE" /><association property="customer" column="CUSTOMERID" select="com.test.mybatis.mapper.MybatiscustomerMapper.getCustomerByID"/> <collection property="itemList" column="ORDERID" javaType="ArrayList" ofType="MybatisOrderItem" select="com.test.mybatis.mapper.MybatisOrderItemMapper.selectItemsByOrderID"/>
</resultMap>
<select id="getOrderByID" resultMap="AssociationSelectMap" parameterType="java.math.BigDecimal" >select *from MYBATISORDER where ORDERID = #{orderid,jdbcType=DECIMAL}
</select>

 MybatisOrderItemMapper.xml:

  <resultMap id="AssociationMap" type="com.test.mybatis.vo.MybatisOrderItem" ><id column="ITEMID" property="itemid" jdbcType="DECIMAL" /><result column="ITEMTYPE" property="itemtype" jdbcType="VARCHAR" /><result column="ITEMQUANTITY" property="itemquantity" jdbcType="VARCHAR" /><result column="ITEMCOST" property="itemcost" jdbcType="VARCHAR" /><association property="order" column="ORDERID" select="com.test.mybatis.mapper.MybatisOrderMapper.getOrderByID"/> </resultMap><select id="selectItemByID" resultMap="AssociationMap" parameterType="java.math.BigDecimal" >select *from MYBATISORDERITEMwhere ITEMID = #{itemid,jdbcType=DECIMAL}</select>

 MybatiscustomerMapper.xml:因为不和别的表做关联,只有简单的id,result配置.

  <resultMap id="BaseResultMap" type="com.test.mybatis.vo.Mybatiscustomer" ><id column="ID" property="id" jdbcType="DECIMAL" /><result column="NAME" property="name" jdbcType="VARCHAR" /></resultMap><select id="getCustomerByID" resultMap="BaseResultMap" parameterType="java.math.BigDecimal" >select *from MYBATISCUSTOMER where ID = #{id,jdbcType=DECIMAL}</select>

 MybatisOrderMapper的getOrderByID会先从MYBATISORDER取数据,然后根据CUSTOMERID调用MybatiscustomerMapper.getCustomerByID,然后根据ORDERID调用MybatisOrderItemMapper.selectItemsByOrderID,对应每一条MYBATISORDER数据,都会分别访问另外两个表各一次.

通过将association和collection的select功能替换为resultMap,再用join方式的SQL可以用一条SQL语句将关联数据取出来:

  <resultMap id="AssociationResultMap" type="com.test.mybatis.vo.MybatisOrder" ><id column="ORDERID" property="orderid" jdbcType="DECIMAL" /><result column="ORDERTYPE" property="ordertype" jdbcType="VARCHAR" /><result column="ORDERDATE" property="orderdate" jdbcType="DATE" /><association property="customer" column="CUSTOMERID" resultMap="com.test.mybatis.mapper.MybatiscustomerMapper.BaseResultMap"/> <collection property="itemList" column="ORDERID" javaType="ArrayList" ofType="com.test.mybatis.vo.MybatisOrderItem" resultMap="com.test.mybatis.mapper.MybatisOrderItemMapper.BaseResultMap"/></resultMap><select id="getOrderAssociation" parameterType="String" resultMap="AssociationResultMap">  SELECT *  FROM mybatisOrder ord LEFT JOIN mybatiscustomer customer ON ord.customerId = customer.ID LEFT JOIN mybatisOrderItem item ON ord.orderid = item.orderid WHERE ord.orderid = #{id}</select>