Question

@gfrizzle指出here LINQ to SQL在给定LINQ时为每个计数操作生成独立的SELECT语句：

From d in db
Group d.a.AuditStatus By d.a.DateCreated Into Group
Select _
    DateIssued = DateCreated, _
    TotalAudits = Group.Count(), _
    TotalCancelled = Group.Count(Function(x) If(x = "Cancelled", True, False)), _
    TotalComplete = Group.Count(Function(x) If(x = "Complete", True, False)), _
    TotalIssued = Group.Count(Function(x) If(x = "Issued", True, False)), _
    TotalPending = Group.Count(Function(x) If (x = "Pending", True, False)), _
    Remaining = 0

由于有问题的数据足够大，这很容易导致巨大的性能损失。我知道我可以将其重写为本机SQL查询，但我在其他地方使用LINQ的几个功能（比如能够破坏我的where子句），如果我可以避免它，我不想放弃它。还有其他解决方法吗？

Answer 1

尤里卡！我终于明白了。这是构建底层SQL的一个逻辑问题。无法将LINQ写为单一传递SQL查询，因为COUNT操作只查看行数，而不管任何WHERE样式子句。

解决方案是使用SUM代替模拟COUNT / WHERE组合。此LINQ查询使用SUM（CASE WHEN）列创建单个传递SQL查询。

    From d In resultSet
    Group d By key = d.a.DateCreated Into g = Group
    Select _
        TotalAudits = g.Sum(Function(x) 1), _
        TotalCancelled = g.Sum(Function(x) If(x.a.AuditStatus = "Cancelled", 1, 0)), _
        TotalComplete = g.Sum(Function(x) If(x.a.AuditStatus = "Complete", 1, 0)), _
        TotalIssued = g.Sum(Function(x) If(x.a.AuditStatus = "Issued", 1, 0)), _
        TotalPending = g.Sum(Function(x) If(x.a.AuditStatus = "Pending", 1, 0)), _
        Remaining = 0

Answer 2

也许如果您使用复合键运行另一个查询来获取子组，它可能会转换为更高效的SQL。我没有使用我的VB Linq语法，所以请考虑这个伪代码：

From d in db 
Group d.a.AuditStatus By New With { d.a.AuditStatus, d.a.DateCreated } Into Group 
Select ...

LINQ用于多次计数操作

2 个答案: