Mybatis詳解2

Mybatis

resultMap的使用

1. 單表查詢

       <resultMap type="orders" id="order_list_result_map">
       <!-- id主鍵的映射， property時候pojo中主鍵的屬性，column：返回結果中主鍵的列-->
       <id property="id" column="id"/>
       <!-- 普通列使用result映射 -->
       <result property="userId" column="user_id"/>
       <result property="number" column="number"/>
       <result property="createtime" column="createtime"/>
       <result property="note" column="note"/>
   </resultMap>
   <select id="getOrderListResultMap" resultMap="order_list_result_map">
       select id,user_id,number,createtime,note from orders
   </select>
   
   <select id="getOrderUserList" resultType="orderuser">
       SELECT
           o.id,
           o.user_id userId,
           o.number,
           o.createtime,
           o.note,
           u.username,
           u.address
       FROM
           orders o
       LEFT JOIN `user` u ON o.user_id = u.id
   </select>

2. 一對一關聯映射

   <resultMap type="orders" id="order_user_resultmap">
       <id property="id" column="id"/>
       <result property="userId" column="user_id"/>
       <result property="number" column="number"/>
       <result property="createtime" column="createtime"/>
       <result property="note" column="note"/>
       <!-- 配置一對一關聯映射 -->
       <!-- property：對於orders中user的屬性.javaType：代表user屬性的數據類型-->
       <association property="user" javaType="com.mydeertrip.mybatis.po.User">
           <id property="id" column="user_id"/>
           <result property="username" column="username"/>
           <result property="address" column="address"/>
       </association>
   </resultMap>
   <select id="getOrderUserResultMap" resultMap="order_user_resultmap">
       SELECT
           o.id,
           o.user_id,
           o.number,
           o.createtime,
           o.note,
           u.username,
           u.address
       FROM
           orders o
       LEFT JOIN `user` u ON o.user_id = u.id
   </select>

3. 一對多關聯映射

       <!-- 一對多關聯映射 -->
   <resultMap type="user" id="user_order_resultmap">
       <id property="id" column="id"/>
       <result property="username" column="username"/>
       <result property="birthday" column="birthday"/>
       <result property="sex" column="sex"/>
       <result property="address" column="address"/>
       <!-- 配置一對多關聯映射 -->
       <!-- property：對於user對象中的集合屬性 -->
       <!-- ofType：集合中每個元素的數據類型 -->
       <collection property="orders" ofType="orders">
           <!-- id對於order的主鍵屬性 -->
           <id property="id" column="oid"/>
           <result property="number" column="number"/>
           <result property="createtime" column="createtime"/>
           <result property="note" column="note"/>
       </collection>
   </resultMap>
   <select id="getUserWithOrders" resultMap="user_order_resultmap">
       SELECT
           u.id,
           u.username,
           u.birthday,
           u.sex,
           u.address,
           o.id oid,
           o.number,
           o.createtime,
           o.note
       FROM
           `user` u
       LEFT JOIN orders o ON u.id = o.user_id
   </select>

動態sql

if標籤

<if test="id!=null">
    and id=#{id}
</if>
<if test="username != null and username != ''">
    and username like '%${username}%'
</if>

if標籤在mapper.xml映射文件中有判斷作用，常用於判斷屬性是否爲空

where標籤

<where>
    <if test="id!=null">
        and id=#{id}
    </if>
    <if test="username != null and username != ''">
        and username like '%${username}%'
    </if>
</where>

where標籤可以將sql語句中多餘的and去掉，避免出現語法錯誤

foreach

    <select id="findUserByIds" parameterType="queryvo" resultType="user">
        SELECT 
            <include refid="user_field_list"/>
         FROM `user` 
        <where>
            <!-- and id in(1,10,20,21,31) -->
            <foreach collection="ids" item="id" open="and id in(" close=")" separator=",">
                #{id}
            </foreach>

        </where>
    </select>

foreach標籤可以遍歷輸入參數中的集合屬性

sql片段

    <sql id="user_field_list">
        id,username,birthday,sex,address
    </sql>  
    <select id="findUserList" parameterType="user" resultType="user">
        select <include refid="user_field_list"/> from user
        <include refid="find_user_list_where"/>
    </select>

用於提取重複的sql語句，必須有id，通過sql標籤定義；引用sql片段時，通過include標籤中的refid屬性引用sql片段，refid的值即爲sql片段的id值