c# - 结果与空 LINQ 查询的联合

Question

在数据库中，我有一些产品和价格。 我需要搜索价格限制。问题是可能有几个限制。例如，0 - 500 $ 1500 - $ 2000

如何合并此 LINQ 查询

var products = from product in db.Products
where product.Price >= 0 and product.Price <= 500

var products = from product in db.Products
where product.Price >= 1500 and product.Price <= 2000

我的变体:

  IQueryable<Deal> allResults = null;
            for(var i = 0; i < price.Length - 1; i = i + 2)
             {
              decimal start,end;
              decimal.TryParse(price[i], out start);
              decimal.TryParse(price[i+1], out end);

              var tempResults = from product in query
                         where (product.DiscountPrice >= start && product.DiscountPrice <=                    end)
                       select product;
                    tempResults.Union(allResults);
                }

如何创建空查询以加入？

Answer 1

一种方法是合并条件:

var products = from product in db.Products
where (product.Price >= 0 and product.Price <= 500)||product.Price >= 1500 and product.Price <= 2000

另一种是使用Union或Concat:

var products = (from product in db.Products
                     where product.Price >= 0 and product.Price <= 500)
         .Union(from product in db.Products
                     where product.Price >= 1500 and product.Price <= 2000);

调整你的代码可以做到:

IQueryable<Deal> allResults = null;
for(var i = 0; i < price.Length - 1; i = i + 2)
{
    decimal start,end;
    decimal.TryParse(price[i], out start);
    decimal.TryParse(price[i+1], out end);

    var tempResults = from product in db.Products
                      where (product.DiscountPrice >= start && product.DiscountPrice <= end)
                      select product;
    if(allResults!=null)
       allResults=allResults.Union(tempResults);
    else
       allResults=tempResults;
}

您不需要一个空查询来合并，您可以只添加一个特殊情况来测试这是否是第一个查询。

但我不太喜欢该代码，因为它混合了两个问题:查询的构造和查询的合并。所以我把它分成两部分:

IEnumerable<Tuple<Decimal,Decimal>> ParsePrices(string[] rawPrices)
{
    for(var i = 0; i < price.Length - 1; i = i + 2)
    {
       decimal start=decimal.Parse(price[i]);
       decimal end  =decimal.Parse(price[i+1]);

       yield return Tuple.Create(start, end);
}

var prices=ParsePrices(rawPrices);
IEnumerable<IQueryable<Deal>> partialQueries=prices.Select( interval=>
       from product in db.Products
       where (product.DiscountPrice >= interval.Item1 && product.DiscountPrice <= interval.Item2)
       select product;);
var mergedQuery=partialQueries.Aggregate((q1,q2)=>q1.Union(q2));