前幾天同事遇到一個問題：ORM框架用的mybatisPlus，分頁插件也用的mybatisPlus自帶的分頁插件,業務是分頁查詢每頁展示十條數據，但測試環境每頁展示的條數是隨機的，第一頁一條，第二頁三條。。。，總之很詭異，具體頁面如下 。看到這個問題感覺還是挺有趣的，但具體代碼沒有參與開發，也不好去直接幫忙去看，直到他把查詢的xml文件發到群里，看了下文件這個查詢用到了一對多，一下就知道原因了（ps 以前踩過的坑啊，所以印象深刻）。
很多情況下會遇到列表頁需要一對多查詢，比如 查詢列表頁是展示各種手機信息，有一列是要展示這種手機所有的內存，比如華為P30有218G,256G,512G,我們先重現下上面問題，具體的表結構和實體類如下
create table TF_L_PHONE
(
ID VARCHAR(32) primary key comment 'ID',
PHONE_BRAND VARCHAR2(60) comment '手機品牌',
PHONE_NAME VARCHAR2(60) comment '手機名稱',
PHONE_CODE VARCHAR2(60) comment '手機編碼',
PHONE_DESC VARCHAR2(240) comment '手機描述',
MARKET_TIME TIMESTAMP comment '手機上市時間'
) comment '手機主表';
create table TF_L_PHONE_RAM
(
ID VARCHAR(32) primary key comment 'ID',
PHONE_ID VARCHAR(32) comment '手機id',
PHONE_RAM number(4) comment '手機內存',
PHONE_FEE number(8) comment '手機價格 單位是分'
) comment '手機內存類型表';
手機主表夠構造14條數據，每條手機構造了2-3條不等的內存表數據。xml文件如下，利用swagger做測試，查詢第一頁，每頁查5條，返回的結果應該是total為14，返回數據5條，但結果卻是total為26，數據3條，我說下問題所在：我們用的分頁插件就是在我們的sql外加上具體數據庫對應的分頁方式（oracle用 RowNum,mysql 用limit）,分頁查詢的時候把每一條多側的數據當成一條分頁數據，那這樣可能查出來的五條都對應同一條主表數據，那這時候映射過后就只有一條數據。記住是先分頁查詢后映射數據結構。
打出來的 sql SELECT * FROM ( SELECT TMP.*, ROWNUM ROW_ID FROM ( select t.ID, t.PHONE_BRAND, t.PHONE_NAME, t.PHONE_CODE, t.PHONE_DESC, t.MARKET_TIME ,PR.id PR_ID,PR.PHONE_ID PR_PHONE_ID,PHONE_RAM,PHONE_FEE from TF_L_PHONE t left JOIN TF_L_PHONE_RAM PR on t.ID = PR.PHONE_ID ) TMP WHERE ROWNUM <=5) WHERE ROW_ID > 0
{
"respCode": "0000",
"respDesc": "success",
"data": {
"page": 1,
"rows": 5,
"total": 26,
"totalPages": 6,
"results": [
{
"id": "100",
"phoneBrand": "榮耀",
"phoneName": "榮耀10",
"phoneCode": "RY10",
"phoneDesc": "榮耀10就是好",
"marketTime": "2019-09-05 11:11:51",
"phoneRamList": [
{
"id": "100",
"phoneId": 100,
"phoneRam": 124,
"phoneFee": 199900
},
{
"id": "101",
"phoneId": 100,
"phoneRam": 256,
"phoneFee": 199900
}
]
},
{
"id": "101",
"phoneBrand": "榮耀",
"phoneName": "榮耀20",
"phoneCode": "RY20",
"phoneDesc": "榮耀20就是好",
"marketTime": "2019-09-05 11:11:51",
"phoneRamList": [
{
"id": "110",
"phoneId": 101,
"phoneRam": 124,
"phoneFee": 199900
},
{
"id": "111",
"phoneId": 101,
"phoneRam": 256,
"phoneFee": 199900
}
]
},
{
"id": "104",
"phoneBrand": "華為",
"phoneName": "P20",
"phoneCode": "HWP20",
"phoneDesc": "華為P20就是好",
"marketTime": "2019-09-05 11:11:52",
"phoneRamList": [
{
"id": "125",
"phoneId": 104,
"phoneRam": 256,
"phoneFee": 299900
}
]
}
]
},
"extend": null
}
現在我們知道問題的原因了，那我們怎么才能查到正確得一對多的分頁結果呢，mybatis提供另一種方式（我理解是子查詢映射）xml文件和結果如下，問題完美解決。
{
"respCode": "0000",
"respDesc": "success",
"data": {
"page": 1,
"rows": 5,
"total": 14,
"totalPages": 3,
"results": [
{
"id": "100",
"phoneBrand": "榮耀",
"phoneName": "榮耀10",
"phoneCode": "RY10",
"phoneDesc": "榮耀10就是好",
"marketTime": "2019-09-05 11:11:51",
"phoneRamList": [
{
"id": "100",
"phoneId": 100,
"phoneRam": 124,
"phoneFee": 199900
},
{
"id": "101",
"phoneId": 100,
"phoneRam": 256,
"phoneFee": 199900
}
]
},
{
"id": "101",
"phoneBrand": "榮耀",
"phoneName": "榮耀20",
"phoneCode": "RY20",
"phoneDesc": "榮耀20就是好",
"marketTime": "2019-09-05 11:11:51",
"phoneRamList": [
{
"id": "110",
"phoneId": 101,
"phoneRam": 124,
"phoneFee": 199900
},
{
"id": "111",
"phoneId": 101,
"phoneRam": 256,
"phoneFee": 199900
}
]
},
{
"id": "102",
"phoneBrand": "榮耀",
"phoneName": "榮耀9",
"phoneCode": "RY9",
"phoneDesc": "榮耀9就是好",
"marketTime": "2019-09-05 11:11:52",
"phoneRamList": [
{
"id": "120",
"phoneId": 102,
"phoneRam": 124,
"phoneFee": 199900
},
{
"id": "121",
"phoneId": 102,
"phoneRam": 256,
"phoneFee": 299900
}
]
},
{
"id": "103",
"phoneBrand": "華為",
"phoneName": "P30",
"phoneCode": "HWP30",
"phoneDesc": "華為P30就是好",
"marketTime": "2019-09-05 11:11:52",
"phoneRamList": [
{
"id": "122",
"phoneId": 103,
"phoneRam": 124,
"phoneFee": 199900
},
{
"id": "123",
"phoneId": 103,
"phoneRam": 256,
"phoneFee": 299900
}
]
},
{
"id": "104",
"phoneBrand": "華為",
"phoneName": "P20",
"phoneCode": "HWP20",
"phoneDesc": "華為P20就是好",
"marketTime": "2019-09-05 11:11:52",
"phoneRamList": [
{
"id": "125",
"phoneId": 104,
"phoneRam": 256,
"phoneFee": 299900
}
]
}
]
},
"extend": null
}