是否可以将此 SQL 查询转换为 linq？

Question

我需要在一个表上计算三个值。在纯 SQL 中，它是这样写的：

select 
    count (*) as num_products,
    sum(case when CreatedAt > '{sql.ToSqlDate(_CreatedAfter)}' then 1 else 0 end) num_new,
    sum(case when UpdatedAt > '{sql.ToSqlDate(_UpdatedAfter)}' then 1 else 0 end)  num_updated 
from 
    Products

在切换到 EF Core 时，我尝试将其转换为 Linq，就像这样

var res = (from p in _db.Products
           let total = _db.Products.Count()
           let NewProducts = _db.Products.Count(s => s.CreatedAt > crDate.Date)
           let UpdatedProducts = _db.Products.Count(s => s.UpdatedAt > updDate.Date) 
           select new { total, NewProducts, UpdatedProducts } );
var response = res.ToList();

但生成的 SQL 查询似乎没有优化

SELECT 
    (SELECT COUNT(*) FROM [Products] AS [p0]) AS [total], 
    (SELECT COUNT(*) FROM [Products] AS [s] 
     WHERE [s].[CreatedAt] > '2019-07-31') AS [NewProducts], 
    (SELECT COUNT(*) FROM [Products] AS [s0] 
     WHERE [s0].[UpdatedAt] > '2019-07-01') AS [UpdatedProducts] 
FROM 
    [Products] AS [p]

也许有人可以帮助将原始 SQL 查询转换为 linq？

tia 是的

Answer 1

对该查询的更直译是：生成一个更有可能在目标表的单次扫描中执行的查询：

var q =
    from p in db.Products
    select new
    {
        p.Id,
        NewProduct = p.CreatedAt > DateTime.Parse("2019-07-31") ? 1 : 0,
        UpdatedProduct = p.UpdatedAt > DateTime.Parse("2019-07-01") ? 1 : 0
    } into counts
    group counts by 1 into grouped
    select new
    {
        ProductCount = grouped.Count(),
        NewProductCount = grouped.Sum(r => r.NewProduct),
        UpdatedProductCount = grouped.Sum(r => r.UpdatedProduct)
    };

意思是这样的：

SELECT COUNT(*) AS [ProductCount], 
       SUM([t].[NewProduct]) AS [NewProductCount], 
       SUM([t].[UpdatedProduct]) AS [UpdatedProductCount]
FROM (
    SELECT [p].[Id], CASE
        WHEN [p].[CreatedAt] > @__Parse_0
        THEN 1 ELSE 0
    END AS [NewProduct], CASE
        WHEN [p].[UpdatedAt] > @__Parse_1
        THEN 1 ELSE 0
    END AS [UpdatedProduct], 1 AS [Key]
    FROM [Products] AS [p]
) AS [t]
GROUP BY [t].[Key]

Answer 2

您的 linq 中不需要 from 子句，因为您不会遍历行。只需使用三个语句：

var total = _db.Products.Count();
var NewProducts = _db.Products.Count(s => s.CreatedAt > crDate.Date);
var UpdatedProducts = _db.Products.Count(s => s.UpdatedAt > updDate.Date) ;