解决字段名与实体类属性名不相同的冲突
1. 准备表和数据：
CREATE TABLE orders(
order_id INT PRIMARY KEY AUTO_INCREMENT,
order_no VARCHAR(20),
order_price FLOAT
);
INSERT INTO orders(order_no, order_price) VALUES('aaaa', 23);
INSERT INTO orders(order_no, order_price) VALUES('bbbb', 33);
INSERT INTO orders(order_no, order_price) VALUES('cccc', 22);
2. 定义实体类：
public class Order {
private int id;
private String orderNo;
private float price;
}
3.查询数据
<select id="selectOrder" parameterType="int" resultType="Order">
select * from orders where order_id=#{id}
</select>
错误：查询结果为null
4.解决方法
方式一: 通过在sql 语句中定义别名
<select id="selectOrder" parameterType="int" resultType="_Order">
select order_id id, order_no orderNo,order_price price from orders where order_id=#{id}
</select>
方式二: 通过<resultMap>
<select id="selectOrderResultMap" parameterType="int" resultMap="orderResultMap">
select * from orders where order_id=#{id}
</select>
<resultMap type="_Order" id="orderResultMap">
<id property="id" column="order_id"/>
<result property="orderNo" column="order_no"/>
<result property="price" column="order_price"/>
</resultMap>
实现关联表查询 一对一关联
<!--
方式一：嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集
封装联表查询的数据(去除重复的数据)
select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=1
-->
<select id="getClass" parameterType="int" resultMap="ClassResultMap">
select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
</select>
<resultMap type="_Classes" id="ClassResultMap">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" javaType="_Teacher">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</association>
</resultMap>
<!--
方式二：嵌套查询：通过执行另外一个SQL 映射语句来返回预期的复杂类型
SELECT * FROM class WHERE c_id=1;
SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id 的值
-->
<select id="getClass2" parameterType="int" resultMap="ClassResultMap2">
select * from class where c_id=#{id}
</select>
<resultMap type="_Classes" id="ClassResultMap2">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" javaType="_Teacher"
select="getTeacher">
</association>
</resultMap>
<select id="getTeacher" parameterType="int" resultType="_Teacher">
SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
</select>
<!--
association:用于一对一关联的查询
property:对象属性的名称
javaType：对象属性的类型
column：所对应的外键字段名
select:使用另一个查询封装的结果
-->