mybatis需要注意的几点

对于mybatis中字段名和model中名不一致的问题：

1.select user_name as username from users;

2.<resultMap id=“userResultMap” type="user">

<id column="user_name" property="username"/>

</resultMap>

3.mybatis 对于字段user_name 到属性 username ，mybatis能自动识别装配 。

<setting name="mapUnderscoreToCamelCase" value="true"/> 
<!--设置启用数据库字段下划线映射到java对象的驼峰式命名属性，默认为false-->

@Param注解

int updateUserById (@Param("id") Integer id,@Param("name") String name);
<update id="updateUserById"> <!--注解之后不需要指定参数-->
  update
    users
  set
    name = #{name}
  where id = #{id}
</update>

int updateUser(Map<String,Object> params);
<update id="updateUser"  parameterTYpe="map"> 
  update
    users
  set
    name = #{name}
  where id = #{id}
</update>

4.<sql ></sql>

<sql id="selectBaseField">
  id as id,
  user_name as userName,
  password as password,
  age as age,
  create_time as createTime
</sql>

<select id = "selUser" resultType="User">
  select
  <include refid="selectBaseField"/>
  from
  users
</select>

5.动态sql

<select id="selectUser" resultType="user">
  select
  <include refid="selectFields"/>
  from
  user
  <where>
    <if test="user_name != null and user_name !='' ">
        and user_name = #{username}          <!--会自动去掉第一个符合条件的 and-->
    </if>
    <if test="age != null and age !='' ">
        and age = #{age}         
    </if>
  </where>
</select>


<update id="updateUser"  parameterTYpe="user"> 
  update
    users
  <set>
     <if test="age != null and age > 18 ">
        and age = #{age} ,        <!--会自动去掉最后一个 ，-->
    </if>
  </set>  
</update>


<insert id="addUser" parameterType="java.util.List">
    insert into users
    (
    user_name,
    age,
    password
    )values
    <foreach collection="list" item="item" separator=","> 
    <!-- <foreach collection="list" item="item" separator="," close=")" open="(">-->
        (
        #{item.username},
        #{item.age},
        #{item.password},
        )
    </foreach>
</insert>

6.关于逻辑分页和物理分页

逻辑分页是一次查询出所有结果，然后对resultSet 结果进行处理，设计算法去分页

物理分页是通过 查表的时候 limit 行数