mybatis 一对多查询collection的两种查询方式

第一种 一次性查询出结果然后封装（该方法不能在主表sql语句分页）
直接用collection标签映射，一次性查询所有记录，
其中tags、roles、files、对应实体类中的成员
查询结果是多条记录，然后mybatis根据主表ID封装
注意：这里不能采用分页查询。因为执行sql查询出的所有记录是未封装的记录。而我们想要的是封装后的分页查询

<resultMap type="KlbDocument" id="KlbDocumentResultById">
        <id     property="documentId"       column="document_id"  />
        <result property="docName"       column="doc_Name"   />
        <result property="categoryId"     column="category_id"   />
        <result property="validity"  column="validity"  />
        <result property="hot"          column="hot"          />
        <result property="comment"       column="comment"       />
        <result property="delFlag"     column="del_flag"     />
        <result property="createTime"       column="create_time"       />
        <result property="updateTime"      column="update_time"     />
        <result property="createBy"      column="create_by"     />
        <result property="updateBy"    column="update_by"   />
        <collection property="tags" ofType="KlbDocTag" >
            <id     property="id"       column="tag_id"  />
            <result property="tagName"       column="tag_name"      />
        </collection>
        <collection  property="roles"  ofType="KlbDocRole" >
            <id     property="id"       column="role_id"  />
            <result property="roleName"       column="role_name"      />
        </collection>
        <collection  property="files"  ofType="KlbFile" >
            <id     property="fileId"       column="file_id"  />
            <result property="fileName"       column="file_name"      />
            <result property="url"       column="url"      />
        </collection>
    </resultMap>
	<select id="selectDocumentById" parameterType="KlbDocument" resultMap="KlbDocumentResultById">
        SELECT d.document_id,d.doc_name,c.category_id,c.cate_name,d.create_time,d.create_by,d.hot,d.validity,dt.id as tag_id,tag.tag_name,dc.id as role_id,r.role_name,file.file_id,file.file_name,file.url
        FROM klb_document d

        LEFT JOIN klb_doc_tag dt ON dt.document_id = d.document_id

        LEFT JOIN klb_tag tag on tag.tag_id = dt.tag_id

        LEFT JOIN klb_file file on file.business_id = d.document_id

        LEFT JOIN klb_doc_role dc on dc.document_id = d.document_id

        LEFT JOIN sys_role r on r.role_id = dc.role_id

        LEFT JOIN klb_category c on c.category_id = d.category_id

        where d.del_flag = '1' and file.source = 'doc' and d.document_id = #{documentId} and d.status = 1
    </select>

第二种 嵌套查询，先查主表，然后根据主表查询结果，查询子表（主表sql可以分页）
先用主表查询，查询出记录，然后遍历记录去子表查询。主表查询时可以加分页。
其中tags、roles、对应实体类中的成员，
collection select对应具体某条查询语句，column中前一个值是定义的变量名 后一个值是主表查询出的结果中的字段
当主表查询需要和子表联查时需要加GROUP BY对查询结果根据主键分组

<resultMap type="KlbDocument" id="KlbDocumentResult">
        <id     property="documentId"       column="document_id"  />
        <result property="docName"       column="doc_Name"   />
        <result property="categoryId"     column="category_id"   />
        <result property="validity"  column="validity"  />
        <result property="hot"          column="hot"          />
        <result property="comment"       column="comment"       />
        <result property="delFlag"     column="del_flag"     />
        <result property="createTime"       column="create_time"       />
        <result property="updateTime"      column="update_time"     />
        <result property="createBy"      column="create_by"     />
        <result property="updateBy"    column="update_by"   />
        <collection property="tags" javaType="java.util.ArrayList" ofType="KlbDocTag"
                    select="com.chinaunicom.system.mapper.base.KlbDocumentMapper.selectTagList" column="{documentId=document_id" />
        <collection property="roles" javaType="java.util.ArrayList" ofType="KlbDocRole"
                    select="com.chinaunicom.system.mapper.base.KlbDocumentMapper.selectRoleList" column="{documentId=document_id" />
    </resultMap>
	<!--根据主表查询，注意如果主表查询是多表联查，需要用GROUP BY 更具主表主键分组，保证主表查询出的记录是不重复的-->
	<select id="selectDocumentList" parameterType="KlbDocument" resultMap="KlbDocumentResult">
       SELECT d.document_id,d.doc_name,d.category_id,c.cate_name,d.create_by,d.hot,d.validity,d.create_time
         FROM klb_document d
         LEFT JOIN klb_doc_tag dt
         ON dt.document_id = d.document_id
         LEFT JOIN klb_tag tag
         ON tag.tag_id = dt.tag_id
         LEFT JOIN klb_file file
         ON file.business_id = d.document_id
         LEFT JOIN klb_doc_role dc
         ON dc.document_id = d.document_id
         LEFT JOIN sys_role r
         ON r.role_id = dc.role_id
         LEFT JOIN klb_category c
         ON c.category_id = d.category_id
         where d.del_flag = '1' and file.source = 'doc' and d.status = 1

        <if test="documentId != null and documentId != 0">
            AND d.document_id = #{documentId}
        </if>
        <if test="docName != null and docName != ''">
            AND d.doc_name like concat('%', #{docName}, '%')
        </if>
        <if test="categoryId != null and categoryId != 0">
            AND d.category_id = #{categoryId}
        </if>
        <if test="createBy != null and createBy != ''">
            AND d.create_by = #{createBy}
        </if>
        <if test="selectTagId != null and selectTagId != 0">
            AND dt.tag_id = #{selectTagId}
        </if>
        <if test="fileSuffix != null and fileSuffix != ''">
            AND file.file_suffix = #{fileSuffix}
        </if>
        <if test="hot != null">
            AND d.hot = #{hot}
        </if>

        <if test="roleId != null and roleId.length > 0">
            AND r.role_id in
            <foreach item="item" collection="roleId" separator="," open="(" close=")" index="">
                #{item}
            </foreach>
        </if>
         GROUP BY d.document_id ORDER BY d.create_time
    </select>
    
    
    <!--根据主表传入主键，去子表查询-->
    <select id="selectTagList" resultType="KlbDocTag">
        SELECT
            dt.tag_id,tag.tag_name
        FROM
            klb_doc_tag dt

        LEFT JOIN klb_tag tag ON tag.tag_id = dt.tag_id
        WHERE dt.document_id = #{documentId}
    </select>
	<!--根据主表传入主键，去子表查询-->
    <select id="selectRoleList" resultType="KlbDocRole">
        SELECT
            dc.role_id,
            r.role_name
        FROM
            klb_doc_role dc
        LEFT JOIN sys_role r ON r.role_id = dc.role_id
        WHERE
            dc.document_id = #{documentId}
    </select>

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版权声明：本文为CSDN博主「杨涛的博客」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/qq_42034205/article/details/120484012