How to translate this SQL query to a LINQ query in EF Core?

c# entity-framework-core linq sql

Question

I have the following table :

Indicators(A INT, B INT, C INT, D INT, TimeInsertedLocal DateTime) . 

And I have the EF Core mapping entity that maps to this table.

I need to translate this SQL query to ef core Linq equivalent query .

SELECT A, B, C, D, TimeInsertedLocal
FROM Indicators
WHERE TimeInsertedLocal >= 
(   
    SELECT MAX(I.TimeInsertedLocal) 
    FROM Indicators AS I
) 

and this is the entity :

public class Indicator
{
    public int A { get; set; }
    public int B { get; set; }
    public int C { get; set; }
    public int D { get; set; }
    public DateTime TimeInsertedLocal { get; set; }
 }

How to write the LINQ query so that EF Core generate the same query or a better query that achieves the same result?

1
2
4/23/2019 5:48:42 PM

Accepted Answer

It's literally one to one translation.

SQL query

SELECT A, B, C, D , TimeInsertedLocal
FROM Indicators
WHERE TimeInsertedLocal >= 
(   
    SELECT MAX(I.TimeInsertedLocal) 
    FROM Indicators AS I
)

EF Core LINQ query:

var indicators = dbContext.Set<Indicator>();
var query = indicators
    .Where(i => i.TimeInsertedLocal >= indicators.Max(i2 => (DateTime?)i2.TimeInsertedLocal));

EF Core generated SQL query:

SELECT [i].[A], [i].[B], [i].[C], [i].[D], [i].[TimeInsertedLocal]
FROM [Indicators] AS [i]
WHERE [i].[TimeInsertedLocal] >= (
    SELECT MAX([i2].[TimeInsertedLocal])
    FROM [Indicators] AS [i2]
)

The only specific detail in LINQ query is the DateTime? cast inside Max, otherwise EF Core will try to emulate LINQ Max method throwing behavior and will evaluate query client side.

4
4/23/2019 5:43:44 PM

Popular Answer

Of course there are no indicators with a value of TimeInsertedLocal that is larger than the largest value of TimeInsertedLocal of all Indicators.

However, it might be that you have several indicators with a value equal to the largest value of TimeInsertedLocal.

If that is the case, you need to make groups of Indicators with the same TimeInsertedLocal, and take the group with the largest value.

var indicatorsWithLargestTimeInsertedLocal = myDbContext.Indicators
    // make groups of Indicators with same TimeInsertedLocal:
    .GroupBy(indicator => indicator.TimeInsertedLocal)

    // put the group with the largest TimeInsertedLocal first:
    .OrderByDescending(group => group.Key)

    // The first group of indicators, is the group with the largest value of TimeInsertedLocal
    .FirstOrDefault();

If you are certain the TimeInsertedLocal is unique, you don't have to GroupBy, there will only be one indicator with the largest TimeInsertedLocal

var indicatorWithLargestTimeInsertedLocal = myDbContext.Indicators
    .OrderByDescending(indicator => indicator.TimeInsertedLocal)
    .FirstOrDefault();


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