MySql unique index over two colums - auto increment [duplicate] - mysql

Question

I have multiple databases with the same structure in which data is sometimes copied across. In order to maintain data integrity I am using two columns as the primary key. One is a database id, which links to a table with info about each database. The other is a table key. It is not unique because it may have multiple rows with this value being the same, but different values in the database_id column.
I am planning on making the two columns into a joint primary key. However I also want to set the table key to auto increment - but based on the database_id column.
EG, With this data:
table_id database_id other_columns
1 1
2 1
3 1
1 2
2 2
If I am adding data that includes the dabase_id of 1 then I want table_id to be automatically set to 4. If the dabase_id is entered as 2 then I want table_id to be automatically set to 3. etc.
What is the best way of achieving this in MySql.

Answer 1

if you are using myisam
http://dev.mysql.com/doc/refman/5.0/en/example-auto-increment.html
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. This is useful
when you want to put data into ordered
groups.
CREATE TABLE animals (
grp ENUM('fish','mammal','bird') NOT NULL,
id MEDIUMINT NOT NULL AUTO_INCREMENT,
name CHAR(30) NOT NULL,
PRIMARY KEY (grp,id)
) ENGINE=MyISAM;
INSERT INTO animals (grp,name) VALUES
('mammal','dog'),('mammal','cat'),
('bird','penguin'),('fish','lax'),('mammal','whale'),
('bird','ostrich');
SELECT * FROM animals ORDER BY grp,id;
Which returns:
+--------+----+---------+
| grp | id | name |
+--------+----+---------+
| fish | 1 | lax |
| mammal | 1 | dog |
| mammal | 2 | cat |
| mammal | 3 | whale |
| bird | 1 | penguin |
| bird | 2 | ostrich |
+--------+----+---------+
For your example:
mysql> CREATE TABLE mytable (
-> table_id MEDIUMINT NOT NULL AUTO_INCREMENT,
-> database_id MEDIUMINT NOT NULL,
-> other_column CHAR(30) NOT NULL,
-> PRIMARY KEY (database_id,table_id)
-> ) ENGINE=MyISAM;
Query OK, 0 rows affected (0.03 sec)
mysql> INSERT INTO mytable (database_id, other_column) VALUES
-> (1,'Foo'),(1,'Bar'),(2,'Baz'),(1,'Bam'),(2,'Zam'),(3,'Zoo');
Query OK, 6 rows affected (0.00 sec)
Records: 6 Duplicates: 0 Warnings: 0
mysql> SELECT * FROM mytable ORDER BY database_id,table_id;
+----------+-------------+--------------+
| table_id | database_id | other_column |
+----------+-------------+--------------+
| 1 | 1 | Foo |
| 2 | 1 | Bar |
| 3 | 1 | Bam |
| 1 | 2 | Baz |
| 2 | 2 | Zam |
| 1 | 3 | Zoo |
+----------+-------------+--------------+
6 rows in set (0.00 sec)

Answer 2

Here's one approach when using innodb which will also be very performant due to the clustered composite index - only available with innodb...
http://dev.mysql.com/doc/refman/5.0/en/innodb-index-types.html
drop table if exists db;
create table db
(
db_id smallint unsigned not null auto_increment primary key,
next_table_id int unsigned not null default 0
)engine=innodb;
drop table if exists tables;
create table tables
(
db_id smallint unsigned not null,
table_id int unsigned not null default 0,
primary key (db_id, table_id) -- composite clustered index
)engine=innodb;
delimiter #
create trigger tables_before_ins_trig before insert on tables
for each row
begin
declare v_id int unsigned default 0;
select next_table_id + 1 into v_id from db where db_id = new.db_id;
set new.table_id = v_id;
update db set next_table_id = v_id where db_id = new.db_id;
end#
delimiter ;
insert into db (next_table_id) values (null),(null),(null);
insert into tables (db_id) values (1),(1),(2),(1),(3),(2);
select * from db;
select * from tables;

Answer 3

you can make the two column primary key unique and the auto-increment key primary.

Answer 4

The solution provided by DTing is excellent and working. But when tried the same in AWS Aurora, it didn't worked and complaining the below error.
Error Code: 1075. Incorrect table definition; there can be only one auto column and it must be defined as a key
Hence suggesting json based solution here.
CREATE TABLE DB_TABLE_XREF (
db VARCHAR(36) NOT NULL,
tables JSON,
PRIMARY KEY (db)
)
Have the first primary key outside, and second primary key inside the json and make second primary key value as auto_incr_sequence.
INSERT INTO `DB_TABLE_XREF`
(`db`, `tables`)
VALUES
('account_db', '{"user_info": 1, "seq" : 1}')
ON DUPLICATE KEY UPDATE `tables` =
JSON_SET(`tables`,
'$."user_info"',
IFNULL(`tables` -> '$."user_info"', `tables` -> '$."seq"' + 1),
'$."seq"',
IFNULL(`tables` -> '$."user_info"', `tables` -> '$."seq"' + 1)
);
And the output is like below
account_db {"user_info" : 1, "user_details" : 2, "seq" : 2}
product_db {"product1" : 1, "product2" : 2, "product3" : 3, "seq" : 3}
If your secondary keys are huge, and afraid of using json, then i would suggest to have stored procedure, to check for MAX(secondary_column) along with lock like below.
SELECT table_id INTO t_id FROM DB_TABLE_XREF WHERE database = db_name AND table = table_name;
IF t_id = 0 THEN
SELECT GET_LOCK(db_name, 10) INTO acq_lock;
-- CALL debug_msg(TRUE, "Acquiring lock");
IF acq_lock = 1 THEN
SELECT table_id INTO t_id FROM DB_TABLE_XREF WHERE database_id = db_name AND table = table_name;
-- double check lock
IF t_id = 0 THEN
SELECT IFNULL((SELECT MAX(table_id) FROM (SELECT table_id FROM DB_TABLE_XREF WHERE database = db_name) AS something), 0) + 1 into t_id;
INSERT INTO DB_TABLE_XREF VALUES (db_name, table_name, t_id);
END IF;
ELSE
-- CALL debug_msg(TRUE, "Failed to acquire lock");
END IF;
COMMIT;