前几天同事遇到一个问题：ORM框架用的mybatisPlus，分页插件也用的mybatisPlus自带的分页插件,业务是分页查询每页展示十条数据，但测试环境每页展示的条数是随机的，第一页一条，第二页三条。。。，总之很诡异，具体页面如下 。看到这个问题感觉还是挺有趣的，但具体代码没有参与开发，也不好去直接帮忙去看，直到他把查询的xml文件发到群里，看了下文件这个查询用到了一对多，一下就知道原因了（ps 以前踩过的坑啊，所以印象深刻）。
很多情况下会遇到列表页需要一对多查询，比如 查询列表页是展示各种手机信息，有一列是要展示这种手机所有的内存，比如华为P30有218G,256G,512G,我们先重现下上面问题，具体的表结构和实体类如下
create table TF_L_PHONE
(
ID VARCHAR(32) primary key comment 'ID',
PHONE_BRAND VARCHAR2(60) comment '手机品牌',
PHONE_NAME VARCHAR2(60) comment '手机名称',
PHONE_CODE VARCHAR2(60) comment '手机编码',
PHONE_DESC VARCHAR2(240) comment '手机描述',
MARKET_TIME TIMESTAMP comment '手机上市时间'
) comment '手机主表';
create table TF_L_PHONE_RAM
(
ID VARCHAR(32) primary key comment 'ID',
PHONE_ID VARCHAR(32) comment '手机id',
PHONE_RAM number(4) comment '手机内存',
PHONE_FEE number(8) comment '手机价格 单位是分'
) comment '手机内存类型表';
手机主表够构造14条数据，每条手机构造了2-3条不等的内存表数据。xml文件如下，利用swagger做测试，查询第一页，每页查5条，返回的结果应该是total为14，返回数据5条，但结果却是total为26，数据3条，我说下问题所在：我们用的分页插件就是在我们的sql外加上具体数据库对应的分页方式（oracle用 RowNum,mysql 用limit）,分页查询的时候把每一条多侧的数据当成一条分页数据，那这样可能查出来的五条都对应同一条主表数据，那这时候映射过后就只有一条数据。记住是先分页查询后映射数据结构。
打出来的 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": "荣耀",
"phoneName": "荣耀10",
"phoneCode": "RY10",
"phoneDesc": "荣耀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": "荣耀",
"phoneName": "荣耀20",
"phoneCode": "RY20",
"phoneDesc": "荣耀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": "华为",
"phoneName": "P20",
"phoneCode": "HWP20",
"phoneDesc": "华为P20就是好",
"marketTime": "2019-09-05 11:11:52",
"phoneRamList": [
{
"id": "125",
"phoneId": 104,
"phoneRam": 256,
"phoneFee": 299900
}
]
}
]
},
"extend": null
}
现在我们知道问题的原因了，那我们怎么才能查到正确得一对多的分页结果呢，mybatis提供另一种方式（我理解是子查询映射）xml文件和结果如下，问题完美解决。
{
"respCode": "0000",
"respDesc": "success",
"data": {
"page": 1,
"rows": 5,
"total": 14,
"totalPages": 3,
"results": [
{
"id": "100",
"phoneBrand": "荣耀",
"phoneName": "荣耀10",
"phoneCode": "RY10",
"phoneDesc": "荣耀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": "荣耀",
"phoneName": "荣耀20",
"phoneCode": "RY20",
"phoneDesc": "荣耀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": "荣耀",
"phoneName": "荣耀9",
"phoneCode": "RY9",
"phoneDesc": "荣耀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": "华为",
"phoneName": "P30",
"phoneCode": "HWP30",
"phoneDesc": "华为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": "华为",
"phoneName": "P20",
"phoneCode": "HWP20",
"phoneDesc": "华为P20就是好",
"marketTime": "2019-09-05 11:11:52",
"phoneRamList": [
{
"id": "125",
"phoneId": 104,
"phoneRam": 256,
"phoneFee": 299900
}
]
}
]
},
"extend": null
}