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 | john@example.com | | 2 | bob@example.com | | 3 | john@example.com | +----+------------------+ 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 | john@example.com | | 2 | bob@example.com | +----+------------------+
DELETE and WHERE clause [Accepted]Algorithm
\nBy joining this table with itself on the Email column, we can get the following code.
\nSELECT p1.*\nFROM Person p1,\n Person p2\nWHERE\n p1.Email = p2.Email\n;\n
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.
SELECT p1.*\nFROM Person p1,\n Person p2\nWHERE\n p1.Email = p2.Email AND p1.Id > p2.Id\n;\n
As we already get the records to be deleted, we can alter this statement to DELETE in the end.
MySQL
\nDELETE p1 FROM Person p1,\n Person p2\nWHERE\n p1.Email = p2.Email AND p1.Id > p2.Id\n