在Linq查询中连接Linq查询的结果-Java 学习之路

我遇到的问题是，查询工作正常，除了查询部分的问题连接，连接查询在查询外工作，但是当我放入其中时它将无法工作并在最后得到错误码 .

var list = (from x in context.Contacts
where !x.DeleteFlag && !x.EmptyFlag
select new models.Contacts.list
{

// CONTACT
Contact = x,

// CONTACT'S PHONE
Phone =
   context.EContacts.Where(e => e.id == x.PrimaryPhoneid)
  .Select(e => e.Title).FirstOrDefault(),

// CONTACT'S EMAIL
Email =
   context.EContacts.Where(e => e.id == x.PrimaryEmailid)
   .Select(e => e.Title).FirstOrDefault(),

// CONTACT'S ACCOUNT
Account =
   context.Accounts.Where(e => e.id == x.Parentid)
   .Select(e => e.AccountName).FirstOrDefault(),

// Problem Is Here With This Query
tag =  string.Concat((from HE in context.HashTagEntities
   join t in context.Accounts on HE.ParentEntityid equals t.id
   where HE.ParentEntityId == 3 &&
   t.AccountName == context.Accounts.Where(e => e.id == x.Parentid).Select(e => e.AccountName)
   .FirstOrDefault()
   from tag in context.HashTags
   where HE.HashTagid == tag.id
   select tag.HashTagText).ToArray()),

}).OrderBy(o => o.Contact.FirstName);

错误：

{“LINQ to Entities无法识别方法'System.String Concat（System.String []）'方法，并且此方法无法转换为存储表达式 . ”} System.SystemException

2 回答

-1

问题正是错误告诉你的 - String.Concat 无法转换为SQL查询 . 因此，将查询分为两部分 - 一部分查询数据库，另一部分在执行concat之前将数据带入内存 .

var listQuery =
        from x in context.Contacts
        where !x.DeleteFlag && !x.EmptyFlag
        orderby x.FirstName
        select new
        {
            Contact = x,
            Phone =
                context.EContacts.Where(e => e.id == x.PrimaryPhoneid)
                .Select(e => e.Title).FirstOrDefault(),
            Email =
                context.EContacts.Where(e => e.id == x.PrimaryEmailid)
                .Select(e => e.Title).FirstOrDefault(),
            Account =
                context.Accounts.Where(e => e.id == x.Parentid)
                .Select(e => e.AccountName).FirstOrDefault(),
            tags =
                from HE in context.HashTagEntities
                join t in context.Accounts on HE.ParentEntityid equals t.id
                where HE.ParentEntityId == 3 &&
                    t.AccountName == context.Accounts.Where(e => e.id == x.Parentid).Select(e => e.AccountName)
                    .FirstOrDefault()
                from tag in context.HashTags
                where HE.HashTagid == tag.id
                select tag.HashTagText,                
        };

    var list =
        from x in listQuery.ToArray()
        select new models.Contacts.list()
        {
            Contact = x.Contact,
            Phone = x.Phone,
            Email = x.Email,
            Account = x.Account,
            tags = String.Concat(x.tags.ToArray()),
        };

回复于 2024-05-09T01:51:23+08:00

你为什么用 string.Contat ？做个总结 .

tag = mainQuery.AccountName + (childQuery.ToArray().Aggregate(x,y)=> x + "," + y)

如果上面的方法没有帮助，我建议你做两个单独的查询 . 首先是获取联系人，第二个是计算HashTags . 然后合并它 . 它将提高性能 .

我认为最好的方法是使用本机T-SQL和实体框架 .

class MyDataRow
{
    public int Id {get;set;}
    public double FieldB {get;set;}
    public string FieldC {get;set;}
}

string queryText = @"
SELECT 
    t1.Id, 
    t1.FieldB, 
    (SELECT hashTag FROM table_2 WHERE contactId = t1.Id) AS FieldC
FROM table_1 AS t1
";

// EF will map properties automatically and you do not have to write and configure stored procedure...
List<MyDataRow> rows = context.ExecuteStoreQuery<MyDataRow>(queryText).ToList();

回复于 2024-05-09T01:51:23+08:00

在Linq查询中连接Linq查询的结果

2 回答

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