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
You can use a filtered unique index for this:
The condition (
LinkedID <> '00000000') takes care of both requirements, i.e. rows with'00000000'orNULLare ignored and not stored in the index, so they are not checked for uniqueness.