CREATE TABLE Activity (activity_name VARCHAR, actID VARCHAR)
CREATE TABLE Participates_in (actID VARCHAR)

What is the name of the activity with the most students?
SELECT T1.activity_name FROM Activity AS T1 JOIN Participates_in AS T2 ON T1.actID = T2.actID GROUP BY T1.actID ORDER BY COUNT(*) DESC LIMIT 1
SELECT "T1"."activity_name" FROM "Activity" AS "T1" JOIN "Participates_in" AS "T2" ON "T1"."actID" = "T2"."actID" GROUP BY "T1"."actID" ORDER BY COUNT(*) DESC NULLS LAST LIMIT 1