var subQuery = from tPeople in People
           join tPet in Pet on tPeople.Id equals tPet.PersonId
           group tPeople by tPeople.Id into g
           select new { Id = g.Key, hasPets = g.Count() };
var mainQuery = from tPeopel in People
                join _subQ in subQuery on tPeopel.Id equals _subQ.Id into _subQLeft
                from subQ in _subQLeft.DefaultIfEmpty()
                join _tCuntry in Country on tPeopel.CountryId equals _tCuntry.Id into _tCountryLeft
                from tCountry in _tCountryLeft.DefaultIfEmpty()     
                select new
                {
                    Id = tPeopel.Id,
                    Age = tPeopel.Age,
                    HasPets= subQ.hasPets,
                    CountryId = tCountry.Id,
                    CountryName = tCountry.Name
                };
var Finalquery = from mainQ in mainQuery
            where mainQ.CountryId == 55
            group mainQ by new { mainQ.CountryId, mainQ.CountryName } into g
            select new
            {
                CountryId = g.Key.CountryId,
                CountyName = g.Key.CountryName,
                First = g.Sum(t => (t.Age < 20 && t.HasPets == 1) ? 1 : 0),
                Second = g.Sum(t => (new int[] { 20, 21, 23, 24, 25 }.Contains(t.Id) && t.HasPets > 0) ? 1 : 0),
                Third = g.Sum(t => t.Age > 25 ? 1 : 0)
            };
var result = await Finalquery.ToListAsync();

为了便于阅读和理解，我把它们分开了
但如果你问我，我建议你用 Raw SQL Query 或者 Dapper 在这种情况下
因为在我看来，它简单、更优化、更快
而且，据我所知，当linq查询（to sql）很复杂时，它不会生成最佳的sql查询
如果你想用 Window Functions() 以及 Sql.Ext. ，我觉得这种情况下它不是很有用，但是如果你想，首先请改变你的想法 SQL Query 使用 window function() 在里面