SQL-9: Identify duplicate values in a SQL table

sql-queries/identify-duplicate-values/functions-and-clauses.sql

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+-- COUNT function
+select COUNT(emp_id) from employee;
+
+-- GROUP BY clause
+select office_location, count(emp_id) from employee group by office_location;
+
+-- HAVING clause
+select office_location, count(emp_id) from employee group by office_location having office_location = 'Palo Alto';

sql-queries/identify-duplicate-values/identify-duplicate-rows.sql

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+-- duplicate values in one column
+SELECT emp_id, COUNT(emp_id)
+FROM employee
+GROUP BY emp_id
+HAVING COUNT(emp_id) > 1;
+
+-- duplicate values in multiple columns
+SELECT emp_id, first_name, last_name, COUNT(*)
+FROM employee
+GROUP BY emp_id, first_name, last_name
+HAVING COUNT(*) > 1;

sql-queries/identify-duplicate-values/sample-dataset.sql

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+CREATE TABLE employee (
+    emp_id INT,
+    first_name VARCHAR(128),
+    last_name VARCHAR(128),
+    email VARCHAR(128),
+    office_location VARCHAR(128)
+);
+
+-- insert all unique records
+INSERT INTO
+    employee (emp_id, first_name, last_name, email, office_location)
+VALUES
+    (1, 'John', 'Doe', 'john.doe@baeldung.com', 'New York'),
+    (2, 'Tom', 'Fisk', 'tom.fisk@baeldung.com', 'Palo Alto'),
+    (3, 'Linda', 'Miller', 'linda.miller@baeldung.com', 'Chicago'),
+    (4, 'Jessica', 'Williams', 'jessica.williams@baeldung.com', 'Palo Alto');
+
+-- insert records with one duplicated column
+INSERT INTO
+    employee (emp_id, first_name, last_name, email, office_location)
+VALUES
+    (1, 'Emma', 'Jackson', 'emma.jackson@baeldung.com', 'Austin'),
+    (3, 'Donald', 'Smith', 'donald.smith@baeldung.com', 'Seattle'),
+    (4, 'Olivia', 'Davis', 'olivia.davis@baeldung.com', 'Boston');
+
+-- insert records with multiple duplicated columns
+INSERT INTO
+    employee (emp_id, first_name, last_name, email, office_location)
+VALUES
+    (2, 'Tom', 'Fisk', 'tom.fisk@baeldung.com', 'Palo Alto'),
+    (3, 'Linda', 'Miller', 'linda.miller@baeldung.com', 'Chicago'),
+    (2, 'Tom', 'Fisk', 'tom.fisk@baeldung.com', 'Palo Alto'),
+    (3, 'Linda', 'Miller', 'linda.miller@baeldung.com', 'Chicago'),
+    (2, 'Tom', 'Fisk', 'tom.fisk@baeldung.com', 'Palo Alto');