why duplicate records in sql code example

Example 1: duplicate records in sql

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

Single field=
SELECT _column, COUNT(*) 
FROM _table
GROUP BY _column
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;