今天写业务代码有这么一个场景，就是在数据库表中插入一条数据，并且这个表中只有这么一条。

因为是用的Mybatis-Plus所以直接用它来实现业务功能，起初是把要插入的数据放到mybatis-plus中，代码如下

LambdaQueryWrapper<ProjectCompany> wrapper = new LambdaQueryWrapper<>();
        wrapper.eq(ProjectCompany::getProjectId,projectId).eq(ProjectCompany::getCompanyId,user.getCompanyId()).eq(ProjectCompany::getIsDelete,"0");
        List<ProjectCompany> projectCompanies = projectCompanyMapper.selectList(wrapper);

LambdaQueryWrapper<ProjectCompany> wrapper = new LambdaQueryWrapper<>();
wrapper.eq(ProjectCompany::getProjectId,projectId).eq(ProjectCompany::getCompanyId,user.getCompanyId()).eq(ProjectCompany::getIsDelete,"0");
List<ProjectCompany> projectCompanies = projectCompanyMapper.selectList(wrapper);

就是把要插入的数据先在表中查出来是否存在，查出来的结果放到List集合中，但是在遍历时始终想不到合适的思路，起初是这样写的

for (ProjectCompany projectCompany1 : projectCompanies) {
            if (!(projectCompany1.getCompanyId().equals(projectCompany.getCompanyId())) && !(projectCompany1.getProjectId().equals(projectCompany.getProjectId()))){
                projectCompanyMapper.insert(projectCompany);
                break;
            }
        }

for (ProjectCompany projectCompany1 : projectCompanies) {
    if (!(projectCompany1.getCompanyId().equals(projectCompany.getCompanyId())) && !(projectCompany1.getProjectId().equals(projectCompany.getProjectId()))){
        projectCompanyMapper.insert(projectCompany);
        break;
    }
}

因为不知道 if (!(projectCompany1.getCompanyId().equals(projectCompany.getCompanyId())) && !(projectCompany1.getProjectId().equals(projectCompany.getProjectId())))   （大家对这个有什么解决办法，欢迎留言给c主提供一下） 该怎么判断。试来试去始终不行，后面就换了一种思路，不查所有数据了，我只在表中查一条

LambdaQueryWrapper<ProjectCompany> wrapper = new LambdaQueryWrapper<>();
        wrapper.eq(ProjectCompany::getProjectId,projectCompany.getProjectId()).eq(ProjectCompany::getCompanyId,projectCompany.getCompanyId()).eq(ProjectCompany::getIsDelete,"0");
ProjectCompany projectCompany1 = projectCompanyMapper.selectOne(wrapper);
            if (projectCompany1 ==null){
                projectCompanyMapper.insert(projectCompany);
            }

LambdaQueryWrapper<ProjectCompany> wrapper = new LambdaQueryWrapper<>();
        wrapper.eq(ProjectCompany::getProjectId,projectCompany.getProjectId()).eq(ProjectCompany::getCompanyId,projectCompany.getCompanyId()).eq(ProjectCompany::getIsDelete,"0");
ProjectCompany projectCompany1 = projectCompanyMapper.selectOne(wrapper);
            if (projectCompany1 ==null){
                projectCompanyMapper.insert(projectCompany);
            }

就是我还用之前的查询条件，只不过这次查询我查一条数据projectCompanyMapper.selectOne(wrapper);

然后判断这个结果是不是空的，如果是空的则说明数据库中没有该数据，则可以进行插入。相反如果不为空，则说明表中有数据。

但是上面代码还会出现一个异常，就是   projectCompanyMapper.selectOne(wrapper);  可能会查出多条数据，所以我们还要对该异常进行一个处理

try {
            ProjectCompany projectCompany1 = projectCompanyMapper.selectOne(wrapper);
            if (projectCompany1 ==null){
                projectCompanyMapper.insert(projectCompany);
            }
        }catch (Exception e){
            throw new BaseException("数据有误，待周一回来调试,原因是ProjectCompany表中");
        }

try {
    ProjectCompany projectCompany1 = projectCompanyMapper.selectOne(wrapper);
    if (projectCompany1 ==null){
        projectCompanyMapper.insert(projectCompany);
    }
}catch (Exception e){
    throw new BaseException("数据有误，待周一回来调试,原因是ProjectCompany表中");
}

因为是周五，要着急下班，所以异常信息自己定义的一个，今天就说这么多吧。

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