IQueryable 查询语句中 OrderByDescending() 扩展方法使用注意点

目的: 查询结构物下所有网关的最新诊断结果.

1. 正确查询 (按诊断时间倒序)

var query = queryDiag.GroupBy(g => new {g.dtuId, g.dtuNo, g.dtuDesp})
                    .Select(s =>
                        s.GroupBy(r => r.diagTime)
                            .Select(e => new
                            {
                                s.Key.dtuId,
                                s.Key.dtuNo,
                                s.Key.dtuDesp,
                                diagTime = e.Key,
                                cpuRatio = e.Where(w => w.diagName == iDAUDiagEntity.CpuRatio)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                memoryRatio = e.Where(w => w.diagName == iDAUDiagEntity.MemoryRatio)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                flashRatio = e.Where(w => w.diagName == iDAUDiagEntity.FlashRatio)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                sdRatio = e.Where(w => w.diagName == iDAUDiagEntity.SdRatio)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                gatewayWorkDuration = e.Where(w => w.diagName == iDAUDiagEntity.GatewayWorkDuration)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                serverWorkDuration = e.Where(w => w.diagName == iDAUDiagEntity.ServerWorkDuration)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                nodesCount = e.Select(l => l.nodesCount).FirstOrDefault()
                            }).OrderByDescending(o => o.diagTime).FirstOrDefault());

查询结果:

结构下每个网关的最新诊断结果.

2. 错误查询 (未实现排序目的)

如果将查询语句中的 ".OrderByDescending()" 调整下位置, 如下代码所示:

var query = queryDiag.GroupBy(g => new {g.dtuId, g.dtuNo, g.dtuDesp})
                    .Select(s =>
                        s.OrderByDescending(o => o.diagTime).GroupBy(r => r.diagTime)
                            .Select(e => new
                            {
                                s.Key.dtuId,
                                s.Key.dtuNo,
                                s.Key.dtuDesp,
                                diagTime = e.Key,
                                cpuRatio = e.Where(w => w.diagName == iDAUDiagEntity.CpuRatio)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                memoryRatio = e.Where(w => w.diagName == iDAUDiagEntity.MemoryRatio)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                flashRatio = e.Where(w => w.diagName == iDAUDiagEntity.FlashRatio)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                sdRatio = e.Where(w => w.diagName == iDAUDiagEntity.SdRatio)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                gatewayWorkDuration = e.Where(w => w.diagName == iDAUDiagEntity.GatewayWorkDuration)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                serverWorkDuration = e.Where(w => w.diagName == iDAUDiagEntity.ServerWorkDuration)
                                    .Select(l => l.diagResult).FirstOrDefault(),
                                nodesCount = e.Select(l => l.nodesCount).FirstOrDefault()
                            }).FirstOrDefault());

查询结果:

没有达到预期结果, 可能是最晚的一条诊断记录, 也可能是历史诊断中的某一条记录.

【小结】

IQueryable 查询语句中 OrderByDescending() 扩展方法使用时需注意：

如果需要将结果集按时间倒序排列, 则需要将 "OrderByDescending()" 扩展方法置后.

置前的话, LINQ语句转换后的SQL语句中没有 "order by" 排序条件, 故此, 达不到倒序的效果.