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Finding Duplicates with SQL

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 )

Modify Columns Size

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:

Alter table table name Modify (column-name datatype)
eg.
Alter table user Modify (name varchar2(200));
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