Все сделал

lesson06/task1.sql

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+use vk;
+
+SET @u_id = 1; #Задаем ID исходного пользователя
+#select @u_id;
+
+select u.id, u.firstname, u.lastname, C
+from (select 
+	  		IF(from_user_id = @u_id,to_user_id,from_user_id) as boltun, #Получаем ID собеседника
+	  		id,
+	  		Count(id) as C	  		
+	  from messages 
+      where from_user_id = @u_id or to_user_id = @u_id
+      group by boltun
+      having C) as a, users as u
+where
+	  a.id = u.id
+limit 1

lesson06/task2.sql

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+use vk;
+
+select Count(*)
+from users as u,media as m,likes as l,profiles as p
+where u.id = m.user_id and
+      m.id = l.media_id and
+      u.id = p.user_id and
+      timestampdiff(year,p.birthday,now()) < 10

lesson06/task3.sql

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+use vk;
+
+select IF(gender = "f","female","male") as gender_win
+from 
+	(select p.gender,Count(*) as count_likes
+	from likes as l 
+			left join 
+		 users as u on l.user_id = u.id 
+		 	inner join
+		 profiles as p on u.id = p.user_id	
+	where 
+    	  p.gender = "f"
+      	union
+	select p.gender,Count(*)
+	from likes as l 
+			left join 
+		 users as u on l.user_id = u.id 
+		 	inner join
+		 profiles as p on u.id = p.user_id
+	where 
+   	   p.gender = "m") as res
+order by count_likes desc
+limit 1