8. We have a schema for a social network We have a collection of users, which is identified by unique holders (short names) Friend tells us that holder1 is a friend of holder2 (Friend is uni-directional: holder2 might not be a friend of holder1) And Message says that holder broadcasted a text (message) from city from city User(holder, name, home_city, bio) Friend(holder1, holder2) Message holder text,from_city) The relational algebra expression below Explain the following SQL query below in English that it represents SELECT X.holder1 FROM Friend X, Friend Y WHERE X.holder2 = Y.holder2 and X.holder1 = Y.holder1 group by X.holder1