Mybatis left join 一對一、一對多、多對多查詢
<!-- 一對一查詢 ,一個訂單對應一個用戶-->
<resultMap type="com.Orders" id="OrdersUserResultMap">
<id column="id" property="id"/>
<result column="user_id" property="userid"/>
<result column="number" property="number"/>
<result column="createtime" property="createTime"/>
<result column="note" property="note"/>
<!-- 訂單關聯用戶 -->
<association property="user" javaType="com.User">
<id column="user_id" property="id"/>
<result column="username" property="username"/>
<result column="sex" property="sex"/>
<result column="address" property="address"/>
</association>
</resultMap>
查詢語句:
<select id="findOrdersUser" resultType="com.OrdersCustom" resultMap="OrdersUserResultMap">
SELECT t1.*, t2.*
FROM orders t1
LEFT JOIN t_user t2
WHERE t1.user_id=t2.id
</select>
一對多查詢(left join 主要是爲了當多方爲null時一方可以查)和多對多
目前有個需求:
條件篩選出一個A表的結果list,另一張B表的uuid,
他們之間的關聯關係是 a.id=b.uuid,
若不存在關聯的B對象則返回null.
因此考慮用左連接,不存在字段也不影響總的A表條數
<resultMap type="com.User" id="userAndItemsResultMap">
<!-- 用戶信息 -->
<id column="user_id" property="id"/>
<result column="username" property="username"/>
<result column="sex" property="sex"/>
<result column="address" property="address"/>
<!-- 一個用戶對應多個訂單 -->
<collection property="ordersList" ofType="com.Orders">
<id column="id" property="id"/>
<result column="user_id" property="userid"/>
<result column="number" property="number"/>
<result column="createtime" property="createTime"/>
<result column="note" property="note"/>
<!-- 一個訂單對應多個訂單明細 -->
<collection property="orderdetails" ofType="com.OrderDetail">
<id column="orderdetail_id" property="id"/>
<result column="items_id" property="itemsId"/>
<result column="items_num" property="itemsNum"/>
<result column="orders_id" property="ordersId"/>
<!-- 一個訂單明細對應一個商品 -->
<association property="items" javaType="com.Items">
<id column="items_id" property="id"/>
<result column="items_name" property="itemsName"/>
<result column="items_detail" property="detail"/>
<result column="items_price" property="price"/>
</association>
</collection>
</collection>
</resultMap>
優化
before:
SELECT
a.name,a.uuid,a.app_name,a.app_uuid,a.release_uuid,a.release_name,
a.layout_template_name,a.layout_template_uuid,a.layout_template_version,
a.cluster_uuid,a.status,a.remark,a.delete_flag,
a.create_time,a.create_user,a.update_time,a.update_user,
d.department_code,d.deploy_type,d.department_name,d.namespace
FROM app_instance as a
LEFT JOIN dms_release as d
ON a.release_uuid = d.uuid
AND d.delete_flag=0
WHERE a.delete_flag=0
ORDER BY a.create_time
DESC LIMIT 1,2
after:
SELECT
a.name,a.uuid,a.app_name,a.app_uuid,a.release_uuid,a.release_name,
a.layout_template_name,a.layout_template_uuid,a.layout_template_version,
a.cluster_uuid,a.status,a.remark,a.delete_flag,
a.create_time,a.create_user,a.update_time,a.update_user,
d.department_code,d.deploy_type,d.department_name,d.namespace
FROM (
select * from app_instance where delete_flag=0
ORDER BY create_time desc LIMIT 1,2
) as a
LEFT JOIN dms_release as d
ON a.release_uuid = d.uuid
AND d.delete_flag=0
創建索引:
ALTER TABLE app_instance ADD INDEX index_release_uuid (release_uuid)
思路:參考文獻
引申改進SQL方案:點此鏈接
1.先篩選出符合條件的再進行join,而不是先全表join再篩選
2.加入關鍵選項的索引
遇到的問題及解決方案:
問題描述: 查詢時父表和子表屬性名一致,查出的數據被先查出的數據覆蓋。
問題原因: 查詢的SQL裏選擇的列中有兩個列名爲id的字段,導致封裝成對象時出問題。
修改方法: 把其中一個列名id使用別名(這裏把role表的列名id使用別名rid)。