mongodb linq查询

最新推荐文章于 2025-05-18 00:55:59 发布

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本文介绍了使用LINQ进行数据查询的几种常见方法，包括验证表中是否有文档、计算数量、去重查询以及选择查询结果中指定位置的元素等操作。

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1.Any() 验证表中是否有文档

var result =
    (from c in collection.AsQueryable<C>()
    select c)
    .Any(c => c.X == 1);
// or
var result =
    collection.AsQueryable<C>()
    .Any(c => c.X == 1);
var result =
    (from c in collection.AsQueryable<C>()
    select c)
    .Any();
// or
var result =
    collection.AsQueryable<C>()
    .Any();

2.count()计算数量

var result =
    (from c in collection.AsQueryable<C>()
    select c)
    .Count();
// or
var result =
    collection.AsQueryable<C>()
    .Count();


With a predicate Count returns the number of documents that match the predicate.
var result =
    (from c in collection.AsQueryable<C>()
    select c)
    .Count(c => c.X == 1);
// or
var result =
    collection.AsQueryable<C>()
    .Count(c => c.X == 1);


var result =
    (from c in collection.AsQueryable<C>()
    where c.X == 1
    select c)
    .Count();
// or
var result =
    collection.AsQueryable<C>()
    .Where(c => c.X == 1)
    .Count();

var result =
    collection.AsQueryable<C>()
    .Select(c => c.X)
    .Count(x => x == 1);

3.Distinct 去重查询

ar result =
    (from c in collection.AsQueryable<C>()
    select c.X)
    .Distinct();
// or
var result =
    collection.AsQueryable<C>()
    .Select(c => c.X)
    .Distinct();


var result =
    (from c in collection.AsQueryable<C>()
    select c.A[i])
    .Distinct();
// or
var result =
    collection.AsQueryable<C>()
    .Select(c => c.A[i])
    .Distinct();

4.ElementAt选择查询结果中指定位置的元素，index要是不存在会报错，ElementAtOrDefault不会保存，index不存在，返回null

var result =
    (from c in collection.AsQueryable<C>()
    where c.X > 0
    orderby c.X
    select c)
    .ElementAt(index);
// or
var result =
    collection.AsQueryable<C>()
    .Where(c => c.X > 0)
    .OrderBy(c => c.X)
    .ElementAt(index);

……未完后续更新