What are common table expressions - [email protected]

sql-queries-8/what-are-common-table-expressions/enhancing-query-readability.sql

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+-- Using a CTE to enhance query readability
+WITH ActiveCourse AS ( 
+    SELECT s.id, s.name AS student_name, 
+            c.name AS course_title, 
+            SUM(c.credits) AS course_credits 
+    FROM Student AS s 
+    JOIN Exam AS e 
+        ON e.student_id = s.id 
+    JOIN course AS c 
+        ON c.id = e.course_id 
+    WHERE c.is_active = 'Yes' 
+    GROUP BY s.id 
+    ) 
+
+    SELECT * FROM ActiveCourse 
+    WHERE course_credits > 70;

sql-queries-8/what-are-common-table-expressions/groupingby-scalar-subselect.sql

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+-- Grouping by a Column Derived from a Scalar Subselect or Non-Deterministic Function
+ITH AgeCategories AS ( 
+        SELECT s.id AS student_id, 
+                s.name AS student_name, 
+                TIMESTAMPDIFF(YEAR, s.birth_date, s.enrollment_date) AS age_at_enrollment, 
+                CASE 
+                        WHEN TIMESTAMPDIFF(YEAR, s.birth_date, s.enrollment_date) = 18 THEN '18 Years' 
+                        WHEN TIMESTAMPDIFF(YEAR, s.birth_date, s.enrollment_date) = 19 THEN '19 Years' 
+                        ELSE 'Above 18' 
+                END AS age_category 
+        FROM Student AS s 
+        ) 
+        SELECT age_category, 
+                COUNT(student_id) AS student_count 
+        FROM AgeCategories 
+                GROUP BY age_category;

sql-queries-8/what-are-common-table-expressions/recursive-cte-queries.sql

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+-- Recursive cte
+ WITH RECURSIVE EnrollmentProgression AS (
+
+    SELECT 
+        id AS student_id,
+        name,
+        YEAR(enrollment_date) AS active_year
+    FROM 
+        Student
+
+    UNION ALL
+
+    -- Recursive statement to increment the year until the graduation year
+    SELECT 
+        ep.student_id,
+        ep.name,
+        ep.active_year + 1 AS year
+    FROM 
+        EnrollmentProgression ep
+    INNER JOIN 
+        student s ON ep.student_id = s.id
+    WHERE 
+        ep.active_year < YEAR(s.graduation_date)
+)
+-- Select all active years for each student's enrollment
+SELECT 
+    student_id,
+    name,
+    active_year
+FROM 
+    EnrollmentProgression
+ORDER BY 
+    student_id, active_year
+LIMIT 20;
+
+
+This gives us a year-by-year breakdown of a student’s active enrollment. The table below shows all th

sql-queries-8/what-are-common-table-expressions/reference-resulting-table-mulltiple-times.sql

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+-- Reference the resulting table multiple times in the same statement
+WITH ExamSummary AS ( 
+    SELECT e.student_id, 
+            s.name AS student_name, 
+            COUNT(*) AS total_exams, 
+            SUM(CASE 
+                    WHEN e.grade = 'A' THEN 1 
+                    ELSE 0 
+                END
+                ) AS a_grades_count, 
+            AVG( CASE 
+                    WHEN e.grade = 'A' THEN 4 
+                    WHEN e.grade = 'B' THEN 3 
+                    WHEN e.grade = 'C' THEN 2 
+                    WHEN e.grade = 'D' THEN 1 
+                    ELSE 0 
+                END ) AS average_grade_points 
+    FROM Exam AS e 
+    JOIN student AS s 
+        ON e.student_id = s.id 
+    GROUP BY e.student_id, s.name 
+    ) 
+    SELECT es1.student_name, 
+            es1.total_exams, 
+            es1.a_grades_count, 
+            es1.average_grade_points, 
+            CASE 
+                WHEN es1.a_grades_count >= 3 THEN 'Top Performer' 
+                ELSE 'Regular Performer' 
+            END AS performance_category 
+    FROM ExamSummary AS es1 
+    JOIN ExamSummary es2 
+        ON es1.student_id = es2.student_id 
+        WHERE es1.total_exams > 3;

sql-queries-8/what-are-common-table-expressions/substitute-for-view.sql

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+-- USING CTE as a substitute for view
+WITH EnrollmentStats AS ( 
+    SELECT YEAR(s.enrollment_date) AS enrollment_year, 
+            AVG(s.gpa) AS average_gpa 
+    FROM Student AS s 
+    WHERE s.gpa IS NOT NULL 
+    GROUP BY YEAR(s.enrollment_date) 
+    ) 
+    SELECT enrollment_year, 
+            ROUND(average_gpa,3) AS average_gpa
+    FROM EnrollmentStats 
+    WHERE average_gpa > 3.0 
+    ORDER BY enrollment_year;