196. Delete Duplicate Emails


Write a SQL query to delete all duplicate email entries in a table named Person, keeping only unique emails based on its smallest Id.

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | [email protected] |
| 2  | [email protected]  |
| 3  | [email protected] |
+----+------------------+
Id is the primary key column for this table.

For example, after running your query, the above Person table should have the following rows:

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | [email protected] |
| 2  | [email protected]  |
+----+------------------+

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Solution

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Approach: Using DELETE and WHERE clause [Accepted]

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Algorithm

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By joining this table with itself on the Email column, we can get the following code.

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SELECT p1.*\nFROM Person p1,\n    Person p2\nWHERE\n    p1.Email = p2.Email\n;\n
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Then we need to find the bigger id having same email address with other records. So we can add a new condition to the WHERE clause like this.

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SELECT p1.*\nFROM Person p1,\n    Person p2\nWHERE\n    p1.Email = p2.Email AND p1.Id > p2.Id\n;\n
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As we already get the records to be deleted, we can alter this statement to DELETE in the end.

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MySQL

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DELETE p1 FROM Person p1,\n    Person p2\nWHERE\n    p1.Email = p2.Email AND p1.Id > p2.Id\n
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