adding unique to existing foreign key - mysql

Question

i have this table
CREATE TABLE IF NOT EXISTS `transaction` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`amount` bigint(20) NOT NULL,
`req_id` int(11) NOT NULL,
`date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`),
KEY `transactions_873a2484` (`req_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_persian_ci AUTO_INCREMENT=914 ;
i want to change this forign key transactions_873a2484 to a unque forign key
basically i want to change it to
UNIQUE KEY `transactions_req_id_de2b5683_uniq` (`req_id`),
i already have lots of data in my table otherwise i would have just remake this table .... is there anyway to do this withouth harming the data ?

Answer 1

I will improve this as I go. MySQL will honor your wishes, even allow you to shoot yourself in the foot as you go:
create table t9
(
id int auto_increment primary key,
thing varchar(20) not null,
key(thing),
unique key (thing),
unique key `yet_another` (thing)
);
-- warning 1831 dupe index
show create table t9;
CREATE TABLE `t9` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`thing` varchar(20) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `thing_2` (`thing`),
UNIQUE KEY `yet_another` (`thing`),
KEY `thing` (`thing`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
So look at all the baggage it you have to carry around with your upserts (read: slow extra unnecessary indexes).
So if you want it as lean as possible, as I mentioned in comments, unwind things first by dropping the FK's in the child tables, the referencing first. See This Answer.
Then drop the current non-unique parent key:
DROP INDEX index_name ON tbl_name;
Then add the unique key in the parent. This is the new referenced:
CREATE UNIQUE INDEX idxName ON tbl_name (colName);
Then add the FK's in the children (the referencing)
CREATE INDEX idxName ON child_tbl_name (colName);
You can get the key names by show create table theTableName or by SHOW INDEX. Use fresh names for the new ones, it doesn't matter.
Such as:
mysql> show index from t9;
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t9 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| t9 | 0 | thing_2 | 1 | thing | A | 0 | NULL | NULL | | BTREE | | |
| t9 | 0 | yet_another | 1 | thing | A | 0 | NULL | NULL | | BTREE | | |
| t9 | 1 | thing | 1 | thing | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

Answer 2

ALTER TABLE `transaction`
DROP INDEX `transactions_873a2484`,
ADD UNIQUE(req_id);
You cannot turn a non-unique into UNIQUE, but the above should do the equivalent. The data will be unharmed.