查询单个表并合并相同日期时间的结果

Question

我有一个表，我在其中插入与 id 相关的值，这些值会随着时间而变化。

示例：

|身份证 |时间戳 |价值 | | 1 | 2018-09-12 02:29:31.154 | 7.139 | | 1 | 2018-09-12 02:40:46.724 | 7.254 | | 2 | 2018-09-06 02:40:46.724 | 132.451 | | 1 | 2018-09-12 02:42:19.841 | 7.645 | | 3 | 2018-09-12 03:01:45.811 | 45.276 | | 1 | 2018-09-12 03:12:59.121 | 7.421 | | 2 | 2018-09-12 03:12:59.121 | 130.789 | | 1 | 2018-09-12 03:15:33.467 | 7.121 | | 2 | 2018-09-12 03:15:33.467 | 136.198 | | 3 | 2018-09-12 03:15:33.467 | 46.971 | | 2 | 2018-09-12 03:27:13.642 | 131.879 | | 3 | 2018-09-12 03:27:13.642 | 44.645 | | 1 | 2018-09-12 03:30:27.564 | 7.691 | | ... | ... | ... |

我的目标是进行单个查询以获取 Id 1、2 和 3 的日期范围之间的值并将它们合并在一起。

结果将是这样的：

{ Timestamp: "2018-09-12 02:29:31.154", id1: 7.123, id2: null, id3: null }
{ Timestamp: "2018-09-12 02:40:46.724", id1: 7.254, id2: 132.451, id3: null }
{ Timestamp: "2018-09-12 02:42:19.841", id1: 7.645, id2: null, id3: null }
{ Timestamp: "2018-09-12 03:01:45.811", id1: null, id2: null, id3: 45.276 }
{ Timestamp: "2018-09-12 03:12:59.121", id1: 7.421, id2: 130.789, id3: null }
{ Timestamp: "2018-09-12 03:15:33.467", id1: 7.121, id2: 136.198, id3: 46.971 }
{ Timestamp: "2018-09-12 03:27:13.642", id1: null, id2: 131.879, id3: 44.645 }
{ Timestamp: "2018-09-12 03:30:27.564", id1: 7.691, id2: null, id3: null }

是否可以在单个查询中进行？

Answer 1

var ids = new List<int> { 1, 2, 3};

var result = (from item in db.Table
              where ids.Contains(item.id) && item.Timestamp > fromDate && item.Timestamp < toDate
              group item by item.Timestamp into sub
              select new 
              {
                  Timestamp = sub.Key,
                  id1 = sub.Where(x => x.id == 1).Select(x => x.Value).FirstOrDefault(),
                  id2 = sub.Where(x => x.id == 2).Select(x => x.Value).FirstOrDefault(),
                  id3 = sub.Where(x => x.id == 3).Select(x => x.Value).FirstOrDefault()
              }).ToList();

Answer 2

如何使用字符串聚合，在 SQL Server 2017 中使用 STRING_AGG() 函数 [documentation] 它会像这样工作：

SELECT [Timestamp]
  -- building string very similar to what you've posted; you can certainly play with this column and format it the way you want
  , STRING_AGG(CONCAT('id', CAST(Id AS nvarchar(50)), ': ', ISNULL(CAST([Value] AS nvarchar(50)), N'NULL')), ', ')
FROM schema.Table
GROUP BY [Timestamp];

如果您使用的是旧版本的 SQL Server，逻辑将是相同的，您只需采用自定义的字符串聚合方式，例如以下其中之一：

• SQL Row concatenation with XML PATH and STUFF

• Optimal way to concatenate/aggregate strings

我自己通常在STRING_AGG()不可用时使用XML解决方案。