c# - 带有 linq 查询语法的条件逻辑

Question

我有以下条件，它执行两个查询之一，这些查询仅由附加子句不同

&& acc.Fb_DataSourceKey == dskId

这是有条件的

var statData = dskId != -1 ? from s in dao.fb_statdata
                            join acc in dao.fb_datasourceadaccount
                            on s.Fb_DataSourceAdAccountId equals acc.Id
                            where s.ReportTypeId == 1
                            && acc.Fb_DataSourceKey == dskId
                            group new { s, acc.Fb_DataSourceKey }  by new { s.Fb_DataSourceAdAccountId, s.Start_time } into res
                            select res
                            :               
                            from s in dao.fb_statdata
                            join acc in dao.fb_datasourceadaccount
                            on s.Fb_DataSourceAdAccountId equals acc.Id
                            where s.ReportTypeId == 1
                            group new { s, acc.Fb_DataSourceKey }  by new { s.Fb_DataSourceAdAccountId, s.Start_time } into res
                            select res;

现在，感谢https://stackoverflow.com/a/2850048/1170932我知道我可以将其更改为以下

            var statData = from s in dao.fb_statdata
                            join acc in dao.fb_datasourceadaccount
                            on s.Fb_DataSourceAdAccountId equals acc.Id
                            where s.ReportTypeId == 1
                            group new { s, acc.Fb_DataSourceKey } by new { s.Fb_DataSourceAdAccountId, s.Start_time } into res
                            select res;

            if (singleDsk)
                statData = from s in statData where s.Key.Fb_DataSourceAdAccountId == dskId select s;

这消除了代码重复，但会导致使用非常低效的外连接（可以理解，真的）。FTR 我们使用 MySQL 作为我们的数据库。

不幸的是，外部连接速度慢得令人无法接受。有没有办法在不生成外连接且不复制代码的情况下执行查询式条件？

Answer 1

在加入之前进行过滤，而不是之后。

var accounts = dao.fb_datasourceadaccount.AsQueryable();

if(dskId != -1)
    accounts = accounts.Where(acc => acc.Fb_DataSourceKey == dskId);

var statData = from s in dao.fb_statdata
                join acc in accounts
                on s.Fb_DataSourceAdAccountId equals acc.Id
                where s.ReportTypeId == 1
                group new { s, acc.Fb_DataSourceKey } 
                by new { s.Fb_DataSourceAdAccountId, s.Start_time } 
                into res
                select res;