how to query for duplicates data in sql code example

Example 1: select duplicates in sql

SELECT username, email, COUNT(*)
FROM users
GROUP BY username, email
HAVING COUNT(*) > 1

Example 2: duplicate a column in sql

ALTER TABLE Table1
ADD SubCategory2 {Type of subcategory 1} {NULL|NOT NULL} 
UPDATE Table1 SET SubCategory2 = SubCategory;