what are common table expressions?

Author: daKeshra

sql-queries-5/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-5/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-5/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;