Finding Duplicates in a table

Here's a handy query for finding duplicates in a table. Suppose you want to find all email addresses in a table that exist more than once:

SELECT email,
COUNT(email) AS NumOccurrences
FROM users
GROUP BY email
HAVING ( COUNT(email) > 1 )

You could also use this technique to find rows that occur exactly once:

SELECT email
FROM users
GROUP BY email
HAVING ( COUNT(email) = 1 )

ROWNUM

Here is the link that explains about ROWNUM concept.

Functions in Oracle

Here is the link to Oracle functions like

1. Trim
2. Length
3. Replace
4. Substr
5. Upper
6. Coalesce
7. Decode
8. Nvl/ Nvl2
9. To_Date
10 To_Number

UNION keywork

Here is a tutorial on how to use union in sql queries.