使用带有逗号分隔 ID 的 LINQ 从 SQL 获取所有数据

Question

I do have a string of Empids separated by comma like:

EMpID:"2007,2008,2002,1992,1000,2108,1085

and I need to retrieve the records of all those specified employees using LINQ query. I tried it with looping but I need to get that in efficient and faster way.

Here goes what i did using looping.

string[] EMpID_str = LeaveDictionary["EMpID"].ToString().Split(',');

for (int i = 0; i < EMpID_str.Length; i++)
            {
                EMpID = Convert.ToInt32(EMpID_str[i]);

               //Linq to get data for each Empid goes here
             }

But What I need is to use single LINQ or Lambda query to retrieve the same.Without looping

Answer 1

If the Ids that you want to fetch are numbers, not strings, then you should not convert the string to an array of strings, but to a sequence of numbers:

IEnumerable<int> employeeIdsToFetch = LeaveDictionary["EMpID"].ToString()
    .Split(',')
    .Select(splitText => Int32.Parse(splitText));

To fetch all employees with thees Ids:

var fetchedEmployees = dbContext.Employees
    .Where(employee => employeeIdsToFetch.Contains(employee.Id))
    .Select(employee => new
    {
         // Select only the employee properties that you plan to use:
         Id = employee.Id,
         Name = employee.Name,
         ...
    });

Answer 2

First convert your,(comma) separated empId to string array like below:

var empArr = EmpId.split(','); 
var employeesResult = emplyeeList.Where(x => empArr.contains(x.EmpId.ToString()));

I hope, it will help someone.

Answer 3

You can use the Expression class to build a Func<int, bool> from your string and use it with the Where methode:

var str = "2,5,8,9,4,6,7";

var para = Expression.Parameter(typeof(int));

var body = str.Split(",")
    .Select(s => int.Parse(s))
    .Select(i => Expression.Constant(i))
    .Select(c => Expression.Equal(para, c))
    .Aggregate((a, b) => Expression.Or(a, b));

Func<int, bool> func = Expression.Lambda<Func<int, bool>>(body, para).Compile();

and if you this solution to work with linq to SQL just dont compile the expression at the end and let the linq to SQL engine compile it to an efficent SQL expression.

Instead of the Aggregate Method (which will produce an expression with linear complexity) one could use an divide and conquer approach to fold the values into one value.

For example with this class:

public static class Helper
{
    public static T EfficientFold<T>(this List<T> list, Func<T, T, T> func)
    {
        return EfficientFold(list, 0, list.Count, func);
    }

    private static T EfficientFold<T>(List<T> list, int lowerbound, int upperbound, Func<T, T, T> func)
    {
        int diff = upperbound - lowerbound;
        var mid = lowerbound + diff / 2;

        if (diff < 1)
        {
            throw new Exception();
        }
        else if (diff == 1)
        {
            return list[lowerbound];
        }
        else
        {
            var left = EfficientFold(list, lowerbound, mid, func);
            var right = EfficientFold(list, mid, upperbound, func);

            return func(left, right);
        }
    }
}

and then we can do

var body = str.Split(",")
    .Select(s => int.Parse(s))
    .Select(i => Expression.Constant(i))
    .Select(c => Expression.Equal(para, c))
    .ToList()
    .EfficientFold((a, b) => Expression.Or(a, b));

which gives the evaluation a complexity of log(n) .