C# Linq 选择最新记录
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Linq to select latest records
提问by Tom Gullen
I have the data structure
我有数据结构
For each item there is a record of it's price on a certain date in each currency. I need to create a query that returns the most current price for each currency.
对于每件商品，都有其在特定日期以每种货币表示的价格的记录。我需要创建一个查询，返回每种货币的最新价格。
This query works, but returns multiple Amountsfor currency ID 1. It should only return 3 records, 7,8 and 9as these represent the most up to date prices in all currencies for this item.
此查询有效，但Amounts为货币 ID返回多个1。它应该只返回 3 条记录，7,8 and 9因为它们代表了该项目所有货币的最新价格。
var q = (from c in db.tblStoreItemPrices where c.ItemID == ID select new { c.CurrencyID, c.Amount });
Please ignore all ordering and assume that records are randomly ordered.
请忽略所有排序并假设记录是随机排序的。
Thanks for any help!
谢谢你的帮助！
采纳答案by Jon
This should work:
这应该有效：
db.tblStoreItemPrices
.Where(c => c.ItemID == ID)
.GroupBy(c => c.CurrencyID)
.Select(g => g.OrderByDescending(c => c.Date).First())
.Select(c => new { c.CurrencyID, c.Amount });
Explanation:
解释：
- Select rows for the specific ItemID
- Group by CurrencyID
- From within each currency group select the row that has the most recent date (leaving one row for for each CurrencyID in the result set)
- Pull out the information you want from these rows
- 为特定的 ItemID 选择行
- 按货币 ID 分组
- 从每个货币组中选择具有最新日期的行（为结果集中的每个 CurrencyID 留一行）
- 从这些行中拉出你想要的信息
