Delete duplicate primary keys from a Mysql database using php programming - php

Question

I have three fields in my database: first name, lastname and email. Email is my primary key. I don't have any other fields in my database.
I need to find a mysql query which can delete duplicated primary keys and their values from the database leaving only one unique email in the database.
I use the following command to display all duplicated primary keys. It worked, but I need to delete all other duplicate entries and keep only one. I am using php programming.
SELECT *
FROM table_name
WHERE primarykey IN (
SELECT primarykey
FROM table_name
GROUP BY primarykey
HAVING count(primarykey) > 1
)
ORDER BY primarykey

Answer 1

Populate a temp table with the ones you want to keep, using GROUP BY, HAVING and MAX on the other columns. Then run your query which deletes too much, then put your copied ones back in. And then make it the actual PK so it doesn't happen again.

Answer 2

You cannot do this with just 1 query, as you will need to use limit and that one needs to be set hard. (You cannot say limit someColumn for example.)
$query = "select primarykey, count(primarykey) as count from table_name group by primarykey having count(primarykey) > 1"
$result = $mysqli->query($query);
while ($row = $result->fetch_assoc()) {
$query = "delete from table_name where primarykey = ? limit " . $row['count'];
$stmt = $mysqli->prepare($query);
$stmt->bind_param('s', $row['primarykey']);
$stmt->execute();
}

Answer 3

Email is not your primary key. Primary keys have a constraint on them where duplicates are not allowed. Your problem, by the way, is just an example of why you want to have an auto-incrementing numeric primary key on all tables. It seems too late for that.
One way to solve your problem is using temporary tables. The idea is to copy the data over, and tehn re-insert it:
create temporary table tmp_emails as
select email, firstname, lastname
from emails
group by email;
truncate table emails;
insert into emails(email, firstname, lastname)
select email, firstname, lastname
from tmp_emails;

Answer 4

Could you try this?
DELETE t1 FROM test t1, test t2
WHERE t1.email = t2.email
AND t1.fn > t2.fn
AND t1.ln > t2.ln;
Here are tests:
CREATE TABLE test
(
email varchar(100),
fn varchar(100),
ln varchar(100)
);
INSERT INTO test VALUES('a#b', 'f1', 'l1');
INSERT INTO test VALUES('a#b', 'f2', 'l2');
INSERT INTO test VALUES('c#d', 'f2', 'l2');
mysql> SELECT * FROM test;
+-------+------+------+
| email | fn | ln |
+-------+------+------+
| a#b | f1 | l1 |
| a#b | f2 | l2 |
| c#d | f2 | l2 |
+-------+------+------+
3 rows in set (0.00 sec)
mysql> DELETE t1 FROM test t1, test t2
-> WHERE t1.email = t2.email
-> AND t1.fn > t2.fn
-> AND t1.ln > t2.ln;
Query OK, 1 row affected (0.00 sec)
mysql> SELECT * FROM test;
+-------+------+------+
| email | fn | ln |
+-------+------+------+
| a#b | f1 | l1 |
| c#d | f2 | l2 |
+-------+------+------+
2 rows in set (0.00 sec)

Answer 5

ALTER TABLE table_name RENAME TO table_name_bak;
CREATE TABLE table_name AS SELECT * FROM table_name_bak LIMIT 0;
ALTER TABLE table_name ADD PRIMARY KEY (email);
INSERT IGNORE INTO table_name SELECT * FROM table_name_bak;