CODEWITHSUNDEEP
mysqltriggersschemaconstraintsmysql-error-1170
Thufir
Thufir

Reputation: 8487

syntax for trigger before insert

Ok, so I see that this the wrong approach:

mysql> 
mysql> show tables;
+---------------------+
| Tables_in_nntp      |
+---------------------+
| articles            |
| newsgroups          |
| newsgroups_articles |
+---------------------+
3 rows in set (0.00 sec)

mysql> describe newsgroups;
+-----------+----------+------+-----+---------+----------------+
| Field     | Type     | Null | Key | Default | Extra          |
+-----------+----------+------+-----+---------+----------------+
| id        | int(11)  | NO   | PRI | NULL    | auto_increment |
| newsgroup | longtext | NO   |     | NULL    |                |
+-----------+----------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> show create table newsgroups;
+------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table      | Create Table                                                                                                                                                                      |
+------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| newsgroups | CREATE TABLE `newsgroups` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `newsgroup` longtext NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=7 DEFAULT CHARSET=latin1 |
+------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> ALTER TABLE newsgroups ADD UNIQUE (newsgroup);
ERROR 1170 (42000): BLOB/TEXT column 'newsgroup' used in key specification without a key length
mysql>

The has should be populated with a trigger?

Ok, as root I made a trigger as so:

mysql> 
mysql> show tables;
+---------------------+
| Tables_in_nntp      |
+---------------------+
| articles            |
| newsgroups          |
| newsgroups_articles |
+---------------------+
3 rows in set (0.00 sec)

mysql> 
mysql> delimiter |
mysql> CREATE TRIGGER make_hash BEFORE INSERT ON newsgroups
    ->   FOR EACH ROW BEGIN
    ->       INSERT INTO hash values ('0');
    ->   END;
    -> |
Query OK, 0 rows affected (0.18 sec)

mysql>

However, that's just dummy data. How can I make that trigger actually create the hash?

Upvotes: 0

Views: 769

Answers (1)

Vatev
Vatev

Reputation: 7590

I think you should leave your primary key as is.
You can add a hash column

ALTER TABLE `newsgroups` ADD COLUMN `hash` CHAR(32) NOT NULL DEFAULT '';

and then populate it with 

UPDATE newsgroups SET hash = MD5(newsgroup);

then remove the duplicates and add your unique constraint.

You can also add BEFORE INSERT and BEFORE UPDATE triggers to set the hash value

CREATE DEFINER=`root`@`localhost` 
TRIGGER `before_insert_newsgroups` 
BEFORE INSERT ON `newsgroups` 
FOR EACH ROW BEGIN

    set new.hash = md5(new.newsgroup);

END

Depending on the SQL client you are using you might wan't to change the DELIMITER before and after the create trigger statement

Upvotes: 1

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