SQL-59: remove duplicates

sql-queries-3/remove-duplicates/remove-duplicates-postgresql.sql

@@ -0,0 +1,26 @@
+-- Inserting Duplicate Records
+INSERT INTO Registration (id, semester, year, course_id, student_id)
+VALUES 
+    (100, 'Fall', 2021, 'CS211', 1001),
+    (200, 'Fall', 2021, 'CS211', 1001),
+    (300, 'Spring', 2022, 'CS212', 1007),
+    (400, 'Spring', 2022, 'CS212', 1007),
+    (500, 'Spring', 2022, 'CS213', 1003),
+    (600, 'Spring', 2022, 'CS213', 1003);
+
+
+-- Deleting Duplicates using the USING clause
+DELETE FROM Registration
+USING (
+    SELECT id
+    FROM (
+        SELECT id,
+            ROW_NUMBER() OVER (
+                PARTITION BY semester, year, course_id, student_id
+                ORDER BY id
+            ) as rn
+        FROM Registration
+    ) t
+    WHERE t.rn > 1
+) duplicate
+WHERE Registration.id = duplicate.id;

sql-queries-3/remove-duplicates/remove-duplicates-sqlserver.sql

@@ -0,0 +1,21 @@
+-- Inserting Duplicate Records
+INSERT INTO Registration (id, semester, year, course_id, student_id)
+VALUES 
+    (100, 'Fall', 2021, 'CS211', 1001),
+    (200, 'Fall', 2021, 'CS211', 1001),
+    (300, 'Spring', 2022, 'CS212', 1007),
+    (400, 'Spring', 2022, 'CS212', 1007),
+    (500, 'Spring', 2022, 'CS213', 1003),
+    (600, 'Spring', 2022, 'CS213', 1003);
+
+-- Deleting Duplicates using a Common Table Expression (CTE)
+WITH COUNTED_DUPLICATES AS (
+    SELECT id,
+        ROW_NUMBER() OVER (
+            PARTITION BY semester, year, course_id, student_id
+            ORDER BY id
+        ) AS rn
+    FROM Registration
+)
+DELETE FROM COUNTED_DUPLICATES
+WHERE rn > 1;

sql-queries-3/remove-duplicates/remove-duplicates.sql

@@ -0,0 +1,59 @@
+-- Inserting Duplicate Records
+INSERT INTO Registration (id, semester, year, course_id, student_id)
+VALUES 
+    (100, 'Fall', 2021, 'CS211', 1001),
+    (200, 'Fall', 2021, 'CS211', 1001),
+    (300, 'Spring', 2022, 'CS212', 1007),
+    (400, 'Spring', 2022, 'CS212', 1007),
+    (500, 'Spring', 2022, 'CS213', 1003),
+    (600, 'Spring', 2022, 'CS213', 1003);
+
+-- Finding Duplicates using GROUP BY and COUNT
+SELECT semester, year, course_id, student_id, COUNT(*) as count
+FROM Registration
+GROUP BY semester, year, course_id, student_id
+HAVING COUNT(*) > 1;
+Select version();
+-- Finding Duplicates using ROW_NUMBER() with PARTITION BY
+SELECT id, semester, year, course_id, student_id,
+    ROW_NUMBER() OVER (
+        PARTITION BY semester, year, course_id, student_id
+        ORDER BY id
+    ) AS rn
+FROM
+    Registration;
+
+-- Deleting Duplicates using Self-Join
+DELETE r1 FROM Registration r1
+INNER JOIN Registration r2
+WHERE
+    r1.semester = r2.semester AND
+    r1.year = r2.year AND
+    r1.course_id = r2.course_id AND
+    r1.student_id = r2.student_id AND
+    r1.id > r2.id;
+
+-- Inserting Duplicate Records
+INSERT INTO Registration (id, semester, year, course_id, student_id)
+VALUES 
+    (100, 'Fall', 2021, 'CS211', 1001),
+    (200, 'Fall', 2021, 'CS211', 1001),
+    (300, 'Spring', 2022, 'CS212', 1007),
+    (400, 'Spring', 2022, 'CS212', 1007),
+    (500, 'Spring', 2022, 'CS213', 1003),
+    (600, 'Spring', 2022, 'CS213', 1003);
+
+-- Deleting Duplicates using Subqueries
+DELETE FROM Registration
+WHERE id IN (
+    SELECT id
+    FROM (
+        SELECT id,
+            ROW_NUMBER() OVER (
+                PARTITION BY semester, year, course_id, student_id
+                ORDER BY id
+            ) as rn
+        FROM Registration
+    ) t
+    WHERE t.rn > 1
+);