使用mybatis結合pageHelper實現分頁查詢，但查詢多表的時候資料就會比真實的要少，如下面程式。

需求是：tab_question是題目表，tab_option是選項表，查詢一道題目下面4個選項，一頁有5個題目。

//controller層
@RestController
@RequestMapping("/question")
public class QuestionController extends BaseController {
    @Autowired
    private QuestionService questionService;
    @GetMapping
    public PageInfo findAll(){
        PageHelper.startPage(1, 5);
        List<Question> questions = questionService.findAll();
        PageInfo pageInfo = new PageInfo(questions);
        return pageInfo;
    }
}

 

mapper配置

<resultMap id="questionMap" type="question">
    <id column="qid" property="qid"/>
    <result column="question" property="question"/>
    <result column="qtype" property="qtype"/>
    <collection property="options" ofType="option" >
        <id column="oid" property="oid"/>
        <result column="optionDec" property="optionDec"/>
        <result column="qid" property="qid"/>
    </collection>
</resultMap>

<select id="findAll" resultMap="questionMap">
    select * from tab_question q left join tab_option o on q.qid = o.qid
</select>
 

查詢結果，一頁只有一個題目，而且第二個題目還不全

{
    "total": 36,
    "list": [
        {
            "qid": 1,
            "question": "這個問題有幾個字",
            "qtype": 1,
            "options": [
                {
                    "oid": 1,
                    "optionDec": "1",
                    "qid": 1
                },
                {
                    "oid": 2,
                    "optionDec": "2",
                    "qid": 1
                },
                {
                    "oid": 3,
                    "optionDec": "3",
                    "qid": 1
                },
                {
                    "oid": 4,
                    "optionDec": "8",
                    "qid": 1
                }
            ]
        },
        {
            "qid": 2,
            "question": "大明王朝1566有哪些角色",
            "qtype": 2,
            "options": [
                {
                    "oid": 5,
                    "optionDec": "嘉靖",
                    "qid": 2
                }
            ]
        }
    ]
}
 

原因是：此時的分頁是多表查詢後得到的記錄再進行分頁（相當於按照選項進行分頁），但實際要查詢的是按照題目進行分頁。

解決辦法：使用子查詢

修改mapper配置，在resultMap中巢狀子查詢

<!--此處省略了id,result標籤-->
<resultMap id="questionMap" type="question">
    <collection property="options" ofType="option" column="qid" select="findOptionById"/>
</resultMap>
<!--主查詢-->
<select id="findAll" resultMap="questionMap">
    select * from tab_question
</select>
<!--子查詢-->
<select id="findOptionById" parameterType="int" resultType="option">
    select * from tab_option where qid = #{qid}
</select>

這次結果就對了

{
    "total": 9,
    "list": [
        {
            "qid": 0,
            "question": "這個問題有幾個字",
            "qtype": 1,
            "options": [
                {
                    "oid": 1,
                    "optionDec": "1",
                    "qid": 1
                },
                {
                    "oid": 2,
                    "optionDec": "2",
                    "qid": 1
                },
                {
                    "oid": 3,
                    "optionDec": "3",
                    "qid": 1
                },
                {
                    "oid": 4,
                    "optionDec": "8",
                    "qid": 1
                }
            ]
        },
        {
            "qid": 0,
            "question": "大明王朝1566有哪些角色",
            "qtype": 2,
            "options": [
                {
                    "oid": 5,
                    "optionDec": "嘉靖",
                    "qid": 2
                },
                {
                    "oid": 6,
                    "optionDec": "海瑞",
                    "qid": 2
                },
                {
                    "oid": 7,
                    "optionDec": "張居正",
                    "qid": 2
                },
                {
                    "oid": 8,
                    "optionDec": "徐階",
                    "qid": 2
                }
            ]
        },
        {
            "qid": 0,
            "question": "海鋼鋒叫什麼",
            "qtype": 1,
            "options": [
                {
                    "oid": 10,
                    "optionDec": "海瑞",
                    "qid": 26
                },
                {
                    "oid": 11,
                    "optionDec": "海筆架",
                    "qid": 26
                },
                {
                    "oid": 12,
                    "optionDec": "潤蓮",
                    "qid": 26
                },
                {
                    "oid": 13,
                    "optionDec": "白圭",
                    "qid": 26
                }
            ]
        },
        {
            "qid": 0,
            "question": "海鋼鋒叫什麼",
            "qtype": 1,
            "options": [
                {
                    "oid": 14,
                    "optionDec": "海瑞",
                    "qid": 28
                },
                {
                    "oid": 15,
                    "optionDec": "海筆架",
                    "qid": 28
                },
                {
                    "oid": 16,
                    "optionDec": "潤蓮",
                    "qid": 28
                },
                {
                    "oid": 17,
                    "optionDec": "白圭",
                    "qid": 28
                }
            ]
        },
        {
            "qid": 0,
            "question": "海鋼鋒叫什麼",
            "qtype": 1,
            "options": [
                {
                    "oid": 18,
                    "optionDec": "海瑞",
                    "qid": 29
                },
                {
                    "oid": 19,
                    "optionDec": "海筆架",
                    "qid": 29
                },
                {
                    "oid": 20,
                    "optionDec": "潤蓮",
                    "qid": 29
                },
                {
                    "oid": 21,
                    "optionDec": "白圭",
                    "qid": 29
                }
            ]
        }
    ]
}