create table Employees
(
   Id int auto_increment
   primary key,
   FirstName text null,
   LastName text null,
   DateOfBirth timestamp not null
);

create table EmployeeEav
(
   Id int auto_increment
   primary key
);

create table EmployeeAttributes
(
  Id int auto_increment
  primary key,
  EmployeeId int not null,
  AttributeName text null,
  AttributeValue text null,
  constraint FK_EmployeeAttributes_EmployeeEav_EmployeeId
  foreign key (EmployeeId) references EmployeeEav (Id)
  on delete cascade
);

create index IX_EmployeeAttributes_EmployeeId
on EmployeeAttributes (EmployeeId);

SELECT MAX(CASE AttributeName
               WHEN 'FirstName'
                   THEN AttributeValue
    END)        AS FirstName,
       MAX(CASE AttributeName
               WHEN 'LastName'
                   THEN AttributeValue
           END) AS LastName,
       MAX(CASE AttributeName
               WHEN 'DateOfBirth'
                   THEN AttributeValue
           END) AS DateOfBirth
FROM efcoresample.EmployeeAttributes
WHERE EmployeeId IN (SELECT EmployeeId
                     FROM efcoresample.EmployeeAttributes
                     WHERE AttributeName = 'DateOfBirth'
                       AND AttributeValue > DATE_SUB(CURRENT_DATE(), INTERVAL 25 YEAR))
  AND AttributeName IN ('FirstName', 'LastName', 'DateOfBirth')
GROUP BY EmployeeId;

string[] columnNames = {"FirstName", "LastName", "DateOfBirth"};
var employees = dbContext.EmployeeAttributes
    .Where(x => 
                dbContext.EmployeeAttributes
                    .Where(i => i.AttributeName == "DateOfBirth")
                    .Select(eId => eId.EmployeeId).Contains(x.EmployeeId) &&
                columnNames.Contains(x.AttributeName))
    .GroupBy(x => x.EmployeeId)
    .Select(g => new
    {
        FirstName = g.Max(f => f.AttributeName == "FirstName" ? f.AttributeValue : ""),
        LastName = g.Max(f => f.AttributeName == "LastName"? f.AttributeValue : ""),
        DateOfBirth = g.Max(f => f.AttributeName == "DateOfBirth"? f.AttributeValue : ""),
        Id = g.Key
    })
    .ToList()
    .Where(x => DateTime.ParseExact(x.DateOfBirth, "yyyy-MM-dd", CultureInfo.InvariantCulture) > DateTime.Now.AddYears(-25));

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