added example for sql server

Author: yadavan88

sql-queries-11/as-alias/as-alias.sql

@@ -4,11 +4,15 @@ FROM Student;
 SELECT name, graduation_date - enrollment_date AS study_duration_in_days 
 FROM Student;
 
+-- ONLY in SQL Server since - operator not supported for date
+SELECT name, DATEDIFF(DAY, enrollment_date, graduation_date) AS study_duration_in_days
+FROM Student;
+
 SELECT s.name,e.course_id,e.grade 
 FROM Student AS s 
 JOIN Exam AS e ON s.id = e.student_id 
 WHERE e.grade IS NOT NULL;
 
 SELECT s.name,s.gpa 
 FROM Student AS s, (SELECT AVG(gpa) AS overall_avg_gpa FROM Student) AS avg_gpa_subquery 
-WHERE s.gpa > avg_gpa_subquery.overall_avg_gpa;
+WHERE s.gpa > avg_gpa_subquery.overall_avg_gpa;