Assume the School database shown an the Appendix. Write the foliowing quenes in $(1)$ telational algebra and $(2)$ SQL$.$ State your asoumptions if the semratics air not cleas. COURSES$($courst id coerst$\_$fame, coure$\_$description, crediss, departmet$\_$code$)$ EVROL I MENTS$($studrm$\_$id$,$ coerve id$,$ semelter, grade$)$ MAJORS$($major$\_$code, major$\_$aame, department$\_$code$)$ DEPARTMENTS$($dejustern code, depatment$\_$mene$)$

$($a$)$ Fisd the names of all stadents who havie sat taken any courses. $(12)$

$($b$)$ Find the course mames ind the namber of stadents who lave taken each coorsw, warted by ber mumber of itudents in descrealing order. $(12$ poents$)$

$($c$)$ Find the average grade for the "Database Management Syucess" coure $($I$2$poista$)$