Defining Composite Key with Auto Increment in MySQL - mysql

Question

Scenario:
I have a table which references two foreign keys, and for each unique combination of these foreign keys, has its own auto_increment column. I need to implement a Composite Key that will help identify the row as unique using combination of these three (one foreign keys and one auto_increment column, and one other column with non-unique values)
Table:
CREATE TABLE `issue_log` (
`sr_no` INT NOT NULL AUTO_INCREMENT ,
`app_id` INT NOT NULL ,
`test_id` INT NOT NULL ,
`issue_name` VARCHAR(255) NOT NULL ,
primary key (app_id, test_id,sr_no)
);
Of course, there has to be something wrong with my query, because of which the error thrown is:
ERROR 1075: Incorrect table definition; there can be only one auto
column and it must be defined as a key
What I am trying to achieve:
I have an Application Table (with app_id as its primary key), each Application has a set of Issues to be resolved, and each Application has multiple number of tests (so the test_id col)
The sr_no col should increment for unique app_id and test_id.
i.e. The data in table should look like:
The database engine is InnoDB.
I want to achieve this with as much simplicity as possible (i.e. avoid triggers/procedures if possible - which was suggested for similar cases on other Questions).

Answer 1

You can't have MySQL do this for you automatically for InnoDB tables - you would need to use a trigger or procedure, or user another DB engine such as MyISAM. Auto incrementing can only be done for a single primary key.
Something like the following should work
DELIMITER $$
CREATE TRIGGER xxx BEFORE INSERT ON issue_log
FOR EACH ROW BEGIN
SET NEW.sr_no = (
SELECT IFNULL(MAX(sr_no), 0) + 1
FROM issue_log
WHERE app_id = NEW.app_id
AND test_id = NEW.test_id
);
END $$
DELIMITER ;

Answer 2

You can do this with myISAM and BDB engines. InnoDB does not support this. Quote from MySQL 5.0 Reference Manual.
For MyISAM and BDB tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column) + 1 WHERE prefix=given-prefix.
http://dev.mysql.com/doc/refman/5.0/en/example-auto-increment.html

Answer 3

I don't fully understand your increment requirement on the test_id column, but if you want an ~autoincrement sequence that restarts on every unique combination of (app_id, test_id), you can do an INSERT ... SELECT FROM the same table, like so:
mysql> INSERT INTO `issue_log` (`sr_no`, `app_id`, `test_id`, `issue_name`) SELECT
IFNULL(MAX(`sr_no`), 0) + 1 /* next sequence number */,
3 /* desired app_id */,
1 /* desired test_id */,
'Name of new row'
FROM `issue_log` /* specify the table name as well */
WHERE `app_id` = 3 AND `test_id` = 1 /* same values as in inserted columns */
This assumes a table definition with no declared AUTO_INCREMENT column. You're essentially emulating autoincrement behavior with the IFNULL(MAX()) + 1 clause, but the manual emulation works on arbitrary columns, unlike the built-in autoincrement.
Note that the INSERT ... SELECT being a single query ensures atomicity of the operation. InnoDB will gap-lock the appropriate index, and many concurrent processes can execute this kind of query while still producing non-conflicting sequences.

Answer 4

You can use a unique composite key for sr_no,app_id & test_id. You cannot use incremental in sr_no as this is not unique.
CREATE TABLE IF NOT EXISTS `issue_log` (
`sr_no` int(11) NOT NULL,
`app_id` int(11) NOT NULL,
`test_id` int(11) NOT NULL,
`issue_name` varchar(255) NOT NULL,
UNIQUE KEY `app_id` (`app_id`,`test_id`,`sr_no`)
) ENGINE=InnoDB ;
I have commented out unique constraint violation in sql fiddle to demonstrate (remove # in line 22 of schema and rebuild schema )

Answer 5

This is what I wanted
id tenant
1 1
2 1
3 1
1 2
2 2
3 2
1 3
2 3
3 3
My current table definition is
CREATE TABLE `test_trigger` (
`id` BIGINT NOT NULL,
`tenant` varchar(255) NOT NULL,
PRIMARY KEY (`id`,`tenant`)
);
I created one table for storing the current id for each tenant.
CREATE TABLE `get_val` (
`tenant` varchar(255) NOT NULL,
`next_val` int NOT NULL,
PRIMARY KEY (`tenant`,`next_val`)
) ENGINE=InnoDB ;
Then I created this trigger which solve my problem
DELIMITER $$
CREATE TRIGGER trigger_name
BEFORE INSERT
ON test_trigger
FOR EACH ROW
BEGIN
UPDATE get_val SET next_val = next_val + 1 WHERE tenant = new.tenant;
set new.id = (select next_val from get_val where tenant=new.tenant);
END$$
DELIMITER ;
This approach will be thread safe also because any insertion for the same tenant will happen sequentially because of the update query in the trigger and for different tenants insertions will happen parallelly.

Answer 6

Just add key(sr_no) on auto-increment column:
CREATE TABLE `issue_log` (
`sr_no` INT NOT NULL AUTO_INCREMENT ,
`app_id` INT NOT NULL ,
`test_id` INT NOT NULL ,
`issue_name` VARCHAR(255) NOT NULL ,
primary key (app_id, test_id,sr_no),
key (`sr_no`)
);

Answer 7

Why don't you try to change the position of declare fields as primary key, since when you use "auto_increment" it has to be referenced as the first. Like in the following example
CREATE TABLE `issue_log` (
`sr_no` INT NOT NULL AUTO_INCREMENT ,
`app_id` INT NOT NULL ,
`test_id` INT NOT NULL ,
`issue_name` VARCHAR(255) NOT NULL ,
primary key (sr_no,app_id, test_id)
);