What is interesting about this situation can be solved using the MyISAM storage.
I answedred a question like this back in April 2012 : How can you have two auto-incremental columns in one table? You need to create one table whose sole purpose is the create sequences of work order for each site
CREATE TABLE site_workorder_seq
(
SiteID int not null,
SiteWorkorderNum int not null auto_increment,
PRIMARY KEY (SiteID,SiteWorkorderNum)
) ENGINE=MyISAM;
Here is a sample loading into this table:
mysql> DROP DATABASE david;
Query OK, 1 row affected (0.01 sec)
mysql> CREATE DATABASE david;
Query OK, 1 row affected (0.00 sec)
mysql> USE david
Database changed
mysql> CREATE TABLE site_workorder_seq
-> (
-> SiteID int not null,
-> SiteWorkorderNum int not null auto_increment,
-> PRIMARY KEY (SiteID,SiteWorkorderNum)
-> ) ENGINE=MyISAM;
Query OK, 0 rows affected (0.01 sec)
mysql> INSERT INTO site_workorder_seq (SiteID) VALUES
-> (1),(1),(2),(3),(3),(3),(3),(4),(4),(4),
-> (5),(5),(4),(2),(2),(2);
Query OK, 16 rows affected (0.00 sec)
Records: 16 Duplicates: 0 Warnings: 0
mysql> SELECT * FROM site_workorder_seq;
+--------+------------------+
| SiteID | SiteWorkorderNum |
+--------+------------------+
| 1 | 1 |
| 1 | 2 |
| 2 | 1 |
| 2 | 2 |
| 2 | 3 |
| 2 | 4 |
| 3 | 1 |
| 3 | 2 |
| 3 | 3 |
| 3 | 4 |
| 4 | 1 |
| 4 | 2 |
| 4 | 3 |
| 4 | 4 |
| 5 | 1 |
| 5 | 2 |
+--------+------------------+
16 rows in set (0.00 sec)
mysql>
Let's look at the the last WorkorderNum from each site
mysql> SELECT SiteID,MAX(SiteWorkorderNum) SiteWorkorderNum
-> FROM site_workorder_seq GROUP BY SiteID;
+--------+------------------+
| SiteID | SiteWorkorderNum |
+--------+------------------+
| 1 | 2 |
| 2 | 4 |
| 3 | 4 |
| 4 | 4 |
| 5 | 2 |
+--------+------------------+
5 rows in set (0.05 sec)
mysql>
Now, suppose you want to get the next SiteWorkorderNum for SiteID 3. You could do this:
INSERT INTO site_workorder_seq (SiteID) VALUES (3);
SELECT MAX(SiteWorkorderNum) INTO @nextworkordernum
FROM site_workorder_seq WHERE SiteID=3;
SELECT @nextworkordernum;
Let's run this and see what happens
mysql> INSERT INTO site_workorder_seq (SiteID) VALUES (3);
Query OK, 1 row affected (0.00 sec)
mysql> SELECT MAX(SiteWorkorderNum) INTO @nextworkordernum
-> FROM site_workorder_seq WHERE SiteID=3;
Query OK, 1 row affected (0.00 sec)
mysql> SELECT @nextworkordernum;
+-------------------+
| @nextworkordernum |
+-------------------+
| 5 |
+-------------------+
1 row in set (0.03 sec)
mysql> SELECT * FROM site_workorder_seq;
+--------+------------------+
| SiteID | SiteWorkorderNum |
+--------+------------------+
| 1 | 1 |
| 1 | 2 |
| 2 | 1 |
| 2 | 2 |
| 2 | 3 |
| 2 | 4 |
| 3 | 1 |
| 3 | 2 |
| 3 | 3 |
| 3 | 4 |
| 3 | 5 |
| 4 | 1 |
| 4 | 2 |
| 4 | 3 |
| 4 | 4 |
| 5 | 1 |
| 5 | 2 |
+--------+------------------+
17 rows in set (0.00 sec)
mysql> SELECT SiteID,MAX(SiteWorkorderNum) SiteWorkorderNum
-> FROM site_workorder_seq GROUP BY SiteID;
+--------+------------------+
| SiteID | SiteWorkorderNum |
+--------+------------------+
| 1 | 2 |
| 2 | 4 |
| 3 | 5 |
| 4 | 4 |
| 5 | 2 |
+--------+------------------+
5 rows in set (0.00 sec)
mysql>
As long as you use this one MyISAM apart from all InnoDB tables, you can generate workordernums per site to your hearts content.
Best Answer
Assuming you store all revisions of the document in a table, an approach would be to not store the revision number but calculate it based on the number of revisions stored in the table.
It is, essentially, a derived value, not something that you need to store.
A window function can be used to calculate the revision number, something like
and you'll need a column something like
change_date
to keep track of the order of the revisions.On the other hand, if you just have
revision
as a property of the document and it indicates "how many times the document has changed", then I would go for the optimistic locking approach, something like:If this updates 0 rows, then there has been intermediate update and you need to inform the user of this.
In general, try to keep your solution as simple as possible. In this case by
update
statement rather than aselect
followed by aninsert
orupdate