Difference Between CTE and Subquery (#209)

Author: oluG4real

sql-queries-5/difference_between_CTE_and_Subquery/correlated_subquery.sql

@@ -0,0 +1,14 @@
+SELECT name
+FROM student s
+WHERE (
+    SELECT COUNT(*)
+    FROM registration r
+    WHERE r.student_id = s.id
+) = (
+    SELECT MAX(course_count)
+    FROM (
+        SELECT COUNT(*) AS course_count
+        FROM registration
+        GROUP BY student_id
+    ) AS subquery
+);

sql-queries-5/difference_between_CTE_and_Subquery/cte_use_case.sql

@@ -0,0 +1,18 @@
+-- View the tables
+SELECT * FROM student LIMIT 3;
+
+SELECT * FROM registration LIMIT 3;
+
+
+-- Let's create the CTE here, named course_count 
+WITH Course_count AS ( 
+    SELECT student_id, COUNT(course_id) AS total_courses 
+    FROM Registration GROUP BY student_id 
+) 
+
+-- select a column from student and CTE (Course_count) table, and filter information 
+SELECT s.name AS student_name, cc.total_courses As courses_registered 
+FROM Student s 
+JOIN Course_count cc ON s.id = cc.student_id 
+WHERE cc.total_courses > 12;
+

sql-queries-5/difference_between_CTE_and_Subquery/multi_row_subquery.sql

@@ -0,0 +1,15 @@
+SELECT name
+FROM student
+WHERE id IN (
+    SELECT student_id
+    FROM registration
+    WHERE course_id IN (
+        SELECT course_id
+        FROM teaching
+        WHERE faculty_id = (
+            SELECT id
+            FROM faculty
+            WHERE name = 'Rory Ross'
+        )
+    )
+);

sql-queries-5/difference_between_CTE_and_Subquery/recursive_cte_query.sql

@@ -0,0 +1,19 @@
+WITH RECURSIVE prerequisite_chain AS (
+    -- Anchor member: Start with the target course
+    SELECT c.id, c.name, p.prerequisite_id
+    FROM course c
+    JOIN prerequisite p ON c.id = p.course_id
+    WHERE c.name = 'Advanced Algorithms'
+    
+    UNION ALL
+    
+    -- Recursive member: Find all prerequisites for the courses in the current level
+    SELECT c.id, c.name, p.prerequisite_id
+    FROM course c
+    JOIN prerequisite_chain pc ON c.id = pc.prerequisite_id
+    JOIN prerequisite p ON c.id = p.course_id
+)
+
+SELECT DISTINCT name
+FROM prerequisite_chain;
+

sql-queries-5/difference_between_CTE_and_Subquery/scalar_subquery.sql

@@ -0,0 +1,4 @@
+SELECT name, gpa,
+       (SELECT AVG(gpa) FROM student) AS avg_gpa
+FROM student
+WHERE gpa > (SELECT AVG(gpa) FROM student);

sql-queries-5/difference_between_CTE_and_Subquery/subquery_use_case.sql

@@ -0,0 +1,12 @@
+SELECT name
+FROM student
+WHERE id IN (
+    SELECT student_id
+    FROM registration
+    WHERE course_id = (
+        SELECT id
+        FROM course
+        WHERE name = 'Introduction to Structural Engineering'
+    )
+);
+