updating row with duplicates value - php

Question

If I have a table with 2 columns , what If I am going to update a column in that table that creates duplicate rows, This table has unique constraint as well, is there any way that if unique row get created while I am updating I can process that row?

Answer 1

Usually adding an SQL inline IF statement with some criteria and optional processing and a self-join to detect duplication will do what you're looking for. The true answer will be specific to your structure, but I will give an example for a table called user with a column called id which is the primary key and SSN which has a unique constraint on it. We'll populate it with 2 users and update one of them to duplicate the first one in the unique ssn column"
CREATE TABLE `test`.`user` (
`id` INT NOT NULL,
`SSN` VARCHAR(45) NULL,
PRIMARY KEY (`id`),
UNIQUE INDEX `ssn_UNIQUE` (`SSN` ASC));
INSERT INTO user VALUES (1, "1234567"), (2, "0123456");
As you have noticed, if I run the following update when another user (where id=1) already has SSN="1234567", then we will have made no updates.
UPDATE user SET SSN="1234567" WHERE id=2;
ERROR 1062 (23000): Duplicate entry '1234567' for key 'ssn_UNIQUE'
However, consider the following instead:
UPDATE user u
LEFT JOIN user AS u2
ON u2.SSN="1234567"
SET u.SSN=IF(
u2.id IS NOT NULL,
CONCAT(u2.SSN, "duplicates", u2.id, "onto", u.id),
"1234567")
WHERE u.id=2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
In the above example, the following scenarios could play out:
If user id=1 already has SSN="1234567", and I run the above update, the result will be:
SELECT * FROM test.user;
+----+--------------------------+
| id | SSN |
+----+--------------------------+
| 2 | 1234567duplicates1onto2 |
| 1 | 1234567 |
+----+--------------------------+
2 rows in set (0.00 sec)
If I try to set instead to "01234567" instead, and I run the same above update, the result will be:
SELECT * FROM test.user;
+----+----------+
| id | SSN |
+----+----------+
| 2 | 01234567 |
| 1 | 1234567 |
+----+----------+
2 rows in set (0.00 sec)
If I had a 3rd user, that user might possibly have the value "1234567duplicates2" if two other users had attempts at setting the value to "1234567" similarly:
SELECT * FROM test.user;
+----+-------------------------+
| id | SSN |
+----+-------------------------+
| 1 | 1234567 |
| 2 | 1234567duplicates1onto2 |
| 3 | 1234567duplicates1onto3 |
+----+-------------------------+
3 rows in set (0.00 sec)
As you can see, the "onto" part allows me to have many duplicates in the same update batch.
To adapt this technique, just change the output of the inline IF to be the formula you would use for processing, and the criteria for the JOIN should be anything to provide duplication detection.
http://dev.mysql.com/doc/refman/5.1/en/control-flow-functions.html