c# - 如何在 linq 的 select 语句中使用 where 子句。（即在财产上。）

Question

我有这个 linq 查询：

var sku = (from a in con.MagentoStockBalances
           join b in con.MFGParts on a.SKU equals b.mfgPartKey
           join c in con.DCInventory_Currents on b.mfgPartKey equals c.mfgPartKey
           where a.SKU != 0 && c.dcKey ==6
           select new
           {
               Part_Number = b.mfgPartNumber,
               Stock = a.stockBalance,
               Recomended = a.RecomendedStock,
               Cato = c.totalOnHandQuantity
           }).ToList();

现在我需要删除 c.dcKey ==6 条件并有这样的东西：

var sku = (from a in con.MagentoStockBalances
           join b in con.MFGParts on a.SKU equals b.mfgPartKey
           join c in con.DCInventory_Currents on b.mfgPartKey equals c.mfgPartKey
           where a.SKU != 0
           select new
           {
               Part_Number = b.mfgPartNumber,
               Stock = a.stockBalance,
               Recomended = a.RecomendedStock,
               Cato = c.totalOnHandQuantity where c.dcKey == 6,
              Kerry = c.totalOnHandQuantity where c.dcKey == 7
           }).ToList();

Answer 1

像这样的东西：

Cato = c.dcKey == 6 ? c.totalOnHandQuantity : 0,
Kerry = c.dcKey == 7 ? c.totalOnHandQuantity : 0

?: 语法称为条件运算符。

Answer 2

我不会添加另一个join，而是在 a 中使用单独的查询let：

from a in con.MagentoStockBalances
join b in con.MFGParts on a.SKU equals b.mfgPartKey
where a.SKU != 0
let cs = con.DCInventory_Currents.Where(c => b.mfgPartKey == c.mfgPartKey)
select new
{
    Part_Number = b.mfgPartNumber,
    Stock = a.stockBalance,
    Recomended = a.RecomendedStock,
    Cato = cs.Single(c => c.dcKey == 6).totalOnHandQuantity 
    Kerry = cs.Single(c => c.dcKey == 7).totalOnHandQuantity 
}

虽然我不知道 LINQ to SQL 将如何处理这样的查询（如果它处理它的话）。

Answer 3

var query= from x in context.a 
       where String.IsNullOrEmpty(param1) || (x.p == param1 && x.i == param2)
        select x;