标签：一对多 log 返回 oct tab ack oss soc 包含
基本的*Mapper.xml文件配置就不熬述了具体可参考：
http://www.mybatis.org/mybatis-3/zh/sqlmap-xml.html
1、sql元素
这个元素可以被用来定义可重用的 SQL 代码段，可以包含在其他语句中。它可以被静态地(在加载参数) 参数化. 不同的属性值通过包含的实例变化. 比如：
<sql id="userColumns"> ${alias}.id,${alias}.username,${alias}.password </sql>
这个 SQL 片段可以被包含在其他语句中，例如：
<select id="selectUsers" resultType="map"> select <include refid="userColumns"><property name="alias" value="t1"/></include>, <include refid="userColumns"><property name="alias" value="t2"/></include> from some_table t1 cross join some_table t2 </select>
属性值可以用于包含的refid属性或者包含的字句里面的属性值，例如：
<sql id="sometable"> ${prefix}Table </sql> <sql id="someinclude"> from <include refid="${include_target}"/> </sql> <select id="select" resultType="map"> select field1, field2, field3 <include refid="someinclude"> <property name="prefix" value="Some"/> <property name="include_target" value="sometable"/> </include> </select>
上面这段SQL首先引用了someinclude然后再someinclude里面又引用了sometable。利用该功能配合if就能动态生成SQL语句了。
2、使用Mybatis优雅的解决N+1查询问题
概括地讲,N+1 查询问题可以是这样引起的:
先举一个简单的例子：客户表customer和car是有一对多的关系，假设有实体类如下:
package com.zealzhangz.entity; import java.io.Serializable; import java.util.List; public class Customer implements Serializable { private static final long serialVersionUID = 7500918067281634276L; private String name; // 名称 private String sex; // 性别 private String idCar; private Car customerCar; public String getName() { return name; } public void setName(String name) { this.name = name; } public String getSex() { return sex; } public void setSex(String sex) { this.sex = sex; } public String getIdCar() { return idCar; } public void setIdCar(String idCar) { this.idCar = idCar; } public Car getCustomerCar() { return customerCar; } public void setCustomerCar(Car customerCar) { this.customerCar = customerCar; } }
如果要获取客户和车辆的信息，一般的做法需要两次查询才能查出结果。当然可以换一种方式，把Car中字段拿到Customer中来，SQL JOIN查询也能查出结果，对于List<car>这种情况JOIN就无能为力了。但是mybatis已经提供了直接优雅
完成这种查询的功能SQL如下：
<?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="CustomerMapper"> <resultMap type="Customer" id="customerMap"> <association property="customerCar" column="idCar" javaType="Car" select="selectCar"/> </resultMap> <select id="selectCustomer" resultMap="customerMap" parameterType="string"> select name,sex,cc.id_car AS idCar from customer c INNER JOIN customer_car cc ON c.pk_id=cc.id_customer where c.pk_id = CAST(#{pk_id} AS UNSIGNED) </select> <select id="selectCar" resultType="Car"> select car_prefix AS carPrefix,car_no AS carNo,vin,car_brand_name AS carBrandName, car_series_mame AS carSeriesMame from car where pk_id = CAST(#{id} AS UNSIGNED) </select> </mapper>
结果如下：
{
"customerCar": {
"carBrandName": "Jeep",
"carNo": "A666666",
"carPrefix": "苏",
"carSeriesMame": "Compass [指南者]",
"vin": "1C4NJCCA9CD645865"
},
"name": "张先生",
"sex": "先生"
}
更复杂的情形有多辆车的情况，这种情况我们稍后考虑。
标签：一对多 log 返回 oct tab ack oss soc 包含
原文地址：http://www.cnblogs.com/maxiaofang/p/7160706.html
