Check out example codes for "select duplicates in sql". It will help you in understanding the concepts better.

Code Example 1

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

Code Example 2

/* Gets reps */
SELECT fieldA, COUNT(*)
FROM tableA
GROUP BY fieldA
HAVING COUNT(*) > 1

/* Use reps to filter results */
SELECT a.*
FROM tableA a
JOIN (
	SELECT fieldA, COUNT(*) as 'count'
	FROM tableA
	GROUP BY fieldA
	HAVING COUNT(*) > 1
) b
ON a.fieldA = b.fieldA

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