Thanks in advance.

PEOPLE(userid, name, group, office) userid; unique username name: ordinary name group: vip, phd, tap, etc... office: a room name or NULL Assume a composite index on the People table above as follows. CREATE INDEX compositelndex ON People(name, office). a. Explain what an index is and how it works b. Give a selection query in SQL that uses this index effectively c. Give a selection query in SQL that does not use this index effectively