Database Interview Question: How to Remove Duplicate Rows

Solution: 
DELETE FROM MyTable
WHERE ID NOT IN
(
SELECT MAX(ID)
FROM MyTable

GROUP BY DuplicateColumn1, DuplicateColumn2, DuplicateColumn3)

DELETE e1 FROM EMPLOYEE e1, EMPLOYEE e2 WHERE e1.name = e2.name AND e1.id > e2.id;

Removing Duplicates Using Table Replacement

mysql> CREATE TABLE tmp SELECT last_name, first_name, sex
    ->                  FROM person_tbl;
    ->                  GROUP BY (last_name, first_name);
mysql> DROP TABLE person_tbl;
mysql> ALTER TABLE tmp RENAME TO person_tbl;

An easy way of removing duplicate records from a table is to add an INDEX or PRIMAY KEY to that table. Even if this table is already available, you can use this technique to remove duplicate records and you will be safe in future as well.

mysql> ALTER IGNORE TABLE person_tbl
    -> ADD PRIMARY KEY (last_name, first_name);

Also refer to http://www.tutorialspoint.com/mysql/mysql-handling-duplicates.htm
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