1.不使用聚合函数sql：
select * from sys_role_data a
left JOIN sys_office b ON a.office_id = b.id
LEFT JOIN sys_role c on a.role_id = c.id WHERE a.del_flag = '0'
如果没有查出结果，则展示无记录
2.使用 聚合函数， 我使用的是group_concat函数，来将结果部分元素聚合
SELECT
a.id AS "id",
a.kind AS "kind",
a.type AS "type",
a.role_id AS "roleId",
b.name AS "officeName",
c.name AS "roleName",
a.office_id AS "officeId",
group_concat(a.data_id) AS "dataId",
group_concat(a.data_name) AS "dataName",
a.useable AS "useable",
a.del_flag AS "delFlag",
a.remarks AS "remarks",
a.create_date AS "createDate",
a.create_by AS "createBy.id",
a.update_date AS "updateDate",
a.update_by AS "updateBy.id"
FROM sys_role_data a
LEFT JOIN
sys_office b
ON a.office_id = b.id
LEFT JOIN
sys_role c
ON a.role_id = c.id
WHERE a.del_flag = '0'
没有查询到结果，但是却显示第一条记录，共一条，即出现了为Null的结果集。
应用在Mybatis中，返回结果为List时，会出现 List不为空，有一个元素为 all Elements are null.size为1，如果有做分页处理，则也会有问题
出现这种情况，则需要对其返回集进行处理，过滤掉null元素，或者将null元素设置为相应的默认值
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sysRoleData.setPage(page);
List<sysroledata> resultList = sysRoleDataDao.findList(sysRoleData);
resultList.removeAll(Collections.singleton(
null
));
page.setList(resultList);
page.setCount(resultList.
size
());</sysroledata>
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处理结果如上图所示
感谢:https://www.2cto.com/database/201806/757632.html