mybatis注意事项

　　

1.如果用注解的方式加载配置CRUD查询的语句时，映射文件中的配置是：

<mapper class="com.day03_mybaits.test3.UserMapper"/>而不是

<mapper resource="com/day03_mybaits/test4/orderMapper.xml"/>

2.除了查询，其他的要session.commit();因为增删改不会自动地提交

3一对一的单表查询中，有两种方式：

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="getClassMap">
SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=#{id}
</select>
<resultMap type="_Classes" id="getClassMap">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" 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="getClass2Map">
SELECT * FROM class WHERE c_id=#{id}
</select>

<select id="getTeacher" parameterType="int" resultType="_Teacher">
SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
</select>

<resultMap type="_Classes" id="getClass2Map">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" select="getTeacher">
</association>
</resultMap>

4.一对多种的表中，结合上面的

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="getClassMap">
select * from class c,teacher t,student s where c.teacher_id=t.t_id and c.c_id=s.class_id and c.c_id=#{id}
</select>

<resultMap type="_Classes" id="getClassMap">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" javaType="_Teacher">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</association>
<collection property="list" ofType="_Student">
<id property="id" column="s_id"/>
<result property="name" column="s_name"/>
</collection>


</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="getClassMap2">
select * from class where c_id=#{id}
</select>
<select id="getTeacher2" resultType="_Teacher">
select t_id id,t_name name from teacher where t_id=#{id}
</select>
<select id="getStudent2" resultType="_Student">
<!--一对多的表中这个地方是class_id -->
SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
</select>
<resultMap type="_Classes" id="getClassMap2">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>

<association property="teacher" column="teacher_id" select="getTeacher2"></association>
<collection property="list" column="c_id" select="getStudent2"></collection>
</resultMap>