如何为此 SQL 查询编写 linq 查询？

Question

如何为此 SQL 查询编写 linq 查询

select c.Name, Count(cb.Id) as Total1, Count(cf.Id) as Total2 
from Company c 
left join CompanyBDetails CB on C.Id = CB.CompanyId
left join CompanyFDetails CF on CF.BankId = CB.Id
group by C.Name

Answer 1

所以你有一个Companies的表，其中每个Company有零个或多个BDetails，每个BDetail都属于一个公司，即外键CompanyId所指的公司：一个简单的一对多关系。

同样，每一个BDetail有零个或多个FDetails，每一个FDetail恰好属于一个BDetail，即外键BDetail所指的BDetail。

对于每家公司，您都需要该公司的名称和该公司的 BDetails 数量。在我看来，您还想要该公司所有 BDetails 的 FDetails 总数。

var result = dbContext.Companies.Select(company => new
{
    Name = company.Name,

    BDetailIds = dbContext.BDetails
        .Where(bDetail => bDetail.CompanyId == company.Id)
        .Select(bDetail => bDetail.Id),
})

中间结果：对于每个公司，您都创建了一个对象，该对象包含公司的名称以及所有 BDetails 的 ID

继续 Linq：计算总数：

.Select(company => new
{
    Name = company.Name,

    // Total1 is the number of BDetails of this Company
    Total1 = company.BDetailIds.Count(),

    // Total2 is the number of FDetails of all BDetails of this Company
    // whenever you need the sub-items of a sequence of items, use SelectMany
    Total2 = company.BDetailIds
        .SelectMany(bDetailId => dbContext.FDetails.Where(fDetail => fDetail.BankId == bDetailId))
        .Count();
});

如果您想要更多的属性而不仅仅是总数：

var result = dbContext.Companies.Select(company => new
{
    // Select the company properties that you plan to use:
    Id = company.Id,
    Name = company.Name,
    ...

    BDetail = dbContext.BDetails
        .Where(bDetail => bDetail.CompanyId == company.Id)
        .Select(bDetail => new
        {
             // Select only the bDetail properties of the company that you plan to use
             Id = bDetail.Id,
             ...

             // not needed, you know the value:
             // CompanyId = bDetail.CompanyId,

             FDetails = dbContext.FDetails
                 .Where(fDetail => fDetail.BankId == bDetail.Id)
                 .Select(fDetail => new
                 {
                      // you know the drill by now: only the properties that you plan to use
                      Id = fDetail.Id,
                      ...
                 })
                 .ToList(),
        })
        .ToList(),
});