来自数据表的 C# LINQ 查询

Question

我正在尝试在我们正在使用的搜索页面上实现 LINQ 查询。从初始查询返回的数据布局如下：

ListID ListName TagId TagValue
1      Name1    1     Tag1
1      Name1    2     Tag2
2      Name2    3     Tag3

基本上，列表是搜索类别，标签是搜索词。我想要做的是带回这个列表，将它分成唯一的列表，然后在页面上显示相关的标签。

我已将数据加载到数据表中，现在我正在尝试将列表名称放入我的 SearchCategories 对象中：

public class SearchCategories
{
    public int SearchCategoryId { get; set; }

    public string SearchCategoryName { get; set; }

    public SearchTags searchTags { get; set; }

    public List<SearchTags> lstSearchTags= new List<SearchTags>();
}

和 SearchTag 类：

public class SearchTags
{
    public int SearchTagId { get; set; }

    public string SearchTagValue { get; set; }
}

然后将标签放入对象的相关列表中。

这是我正在使用的代码：

dtLists.Load(reader);

var distinctValues = dtLists.AsEnumerable()
   .Select(row => new SearchCategories
   {
   SearchCategoryId = row.Field<int>("ListNameID"),
   SearchCategoryName = row.Field<string>("listName")
   })
   .Distinct();

较新的 LINQ，但认为这应该更容易。提前致谢。

Answer 1

这样就可以了：

var result = dtLists.AsEnumerable()
        .GroupBy(m => new
        {
            ListID = m.Field<int>("ListID"),
            ListName = m.Field<string>("ListName")
        }, m => m)
        .Select(m => new
        {
            m.Key.ListID,
            m.Key.ListName,
            Tags = m.Select(x => new { x.Field<int>("TagId"), x.Field<string>("TagValue") })
        });

那你当然要映射到你的类型...

Answer 2

使用GroupBy 应该很容易做到这一点。

dtLists.AsEnumerable()
    .GroupBy(x => new { ListNameID = x.Field<int>("ListNameID"), ListName = x.Field<string>("ListName") })
    .Select(x => new SearchCategories()
    {
        SearchCategoryId = x.Key.ListNameID,
        SearchCategoryName = x.Key.ListName,
        lstSearchTags = x.Select(y => new SearchTags(){ SearchTagId = y.Field<int>("TagId"), SearchTagValue = y.Field<string>("TagValue") }).ToList()
    });

Answer 3

我不清楚你想要你的输出，基于哪些标准，只是问你是否想要更精确。

void Main()
{
    var list = new List<Data>(){
                            new Data
                            {
                               ListId = 1,
                               ListName = "Name1",
                               TagId = 1,
                               TagValue = "Tag1"
                            },
                            new Data
                            {
                               ListId = 1,
                               ListName = "Name1",
                               TagId = 2,
                               TagValue = "Tag2"
                            },
                            new Data
                            {
                               ListId = 2,
                               ListName = "Name2",
                               TagId = 3,
                               TagValue = "Tag3"
                            }};

    //this is to group all SearchCategories by SearchCategoryName
    var res = list.Select(x => new SearchCategories
    {
       SearchCategoryId = x.ListId,
       SearchCategoryName = x.ListName
    }).GroupBy(x => x.SearchCategoryName);

    //to have just distinct categories one time
    var res2 = list.Select(x => new SearchCategories
    {
        SearchCategoryId = x.ListId,
        SearchCategoryName = x.ListName
    }).Distinct(new SearchCategoriesComparer());

    //display your data here based on a Key. The output should be Key=Name1  with two elements inside and the other one with one element Key=Name2
    res.Dump(); 

    //display your data here
    res2.Dump(); 

}

public class Data
{
    public int ListId   {get; set;}
    public string ListName { get; set; }
    public int TagId { get; set; }
    public string TagValue { get; set;}
}


public class SearchCategories
{
    public int SearchCategoryId { get; set; }

    public string SearchCategoryName { get; set; }

    public List<SearchTags> lstSubCategories = new List<SearchTags>();
}

public class SearchCategoriesComparer : IEqualityComparer<SearchCategories>
{
    public bool Equals(SearchCategories x, SearchCategories y)
    {
        return x.SearchCategoryName == y.SearchCategoryName;
    }

    public int GetHashCode(SearchCategories obj)
    {
        return obj.SearchCategoryName.GetHashCode();
    }
}


public class SearchTags
{
    public string Tag{ get; set;}
}

Answer 4

尝试以下类似的方法，使用 GroupBy，不确定您的 SearchTags 是什么，因此将 x => new SearchTags(x) 替换为它的构造方式。

dtLists.AsEnumerable()
    .Select(row => new {
       SearchCategoryId = row.Field<int>("ListID "),
       SearchCategoryName = row.Field<string>("listName"),
       TagId = row.Field<int>("TagId"),
       TagValue = row.Field<string>("TagValue")
    })
    .GroupBy(x => x.SearchCategoryId)
    .Select(g => new SearchCategories() {
                SearchCategoryId  = g.Key,
                SearchCategoryName = g.First().SearchCategoryName,
                lstSubCategories = g.Select(x => new SearchTags(){ SearchTagId = x.TagId, SearchTagValue = x.TagValue}).ToList()
                }).ToList();