子查詢：
同表中子查詢直接在statement中寫，和sql格式一樣；
不同表間的子查詢通過resultMap來實作：
<resultMap type = "Book" id = "BookAndReader" autoMapping = "true">
<id colume = "id" property = "id"/>
<association property = "reader" javaType = "Reader" autoMapping = "true" select = "queryReaderById" column = "user_id">
<!--select進行子查詢宣告，其引數為子查詢陳述句ID-->
<!--column為子查詢引數，即從pojo類傳過來的#{id}-->
</association>
</resultMap>
<select id = "queryReaderAll" resultMap = "BookAndReader">
select * from tb_order where order_number = #{number}
</select>
<select id = "queryReaderById" resultType = "Reader">
<!--這里是子查詢物體-->
select * from tb_user where id = #{id}
</select>
這里應該是主查詢先執行
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