score:0

var result = (from a in db.table_1
              from b in db.table_2 
               where a.id == b.id
               && db.state.any(c=>c.regionid  == 1 && b.stateid == c.stateid) 
                select new
                {
                   a.id
                }).tolist();

score:0

where db.states.any(x => x.regionid == 1 &&  x.stateid == b.stateid)
select...

score:0

var var1 = (from a in db.table_1
            join b in db.table_2
            on a.id equals b.id
            where db.state.any(c=> c.regionid == 1 && c.stateid == b.stateid)
            select new
            {
               a.id
            }).tolist();)

score:0

var var1 = (from a in db.table_1
                join b in db.table_2
                on a.id equals b.id
                where (db.state.where(x => x.regionid == 1).select(x => 
                       x.stateid)).contains(b.id)
                select new
                {
                   a.id
                }).tolist();)

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