auto increment using 'case' in mysql - mysql

Question

I have a simple table in mysql that has different types of records, differentiated by a values in the column ptype
my table looks like this
id1...ptype..usr...item
1.....43.......2......7001
2.....44.......2......8001
3.....43.......2......7002
4.....43.......2......7003
5.....43.......3......7001
When I add a new record, I need my query to insert an auto incremented value in the item column, based upon ptype and specific to usr. i.e. if i insert a new record
id1...ptype..usr...item
6.....43.......3......?
it would add 1 to the highest number existing for ptype=43 and usr=3
id1...ptype..usr...item
6.....43.......3......7002
if we added another record for ptype=44 and usr=2
id1...ptype..usr...item
7.....44.......2......8002
i think i should do this by initially inserting the new record with item blank and then updating that record with information derived from the new record(i.e. #lastid) using the CASE WHEN THEN method, but it's not working.
SET #lastid := LAST_INSERT_ID();
SET #ptype = (SELECT `ptype` FROM a1 WHERE `id1` = #lastid);
SET #item = (SELECT (
CASE
when #ptype = 41 then (SELECT 1 + coalesce((SELECT max(`item`) FROM `a1` WHERE `ptype` = 41 AND `plate`=7 AND `userid` = #userid), 5000))
when #ptype = 42 then (SELECT 1 + coalesce((SELECT max(`item`) FROM `a1` WHERE `ptype` = 42 AND `plate`=7 AND `userid` = #userid), 6000))
when #ptype = 43 then (SELECT 1 + coalesce((SELECT max(`item`) FROM `a1` WHERE `ptype` = 43 AND `plate`=7 AND `userid` = #userid), 7000))
when #ptype = 44 then (SELECT 1 + coalesce((SELECT max(`item`) FROM `a1` WHERE `ptype` = 44 AND `plate`=7 AND `userid` = #userid), 8000))
when #ptype = 45 then (SELECT 1 + coalesce((SELECT max(`item`) FROM `a1` WHERE `ptype` = 45 AND `plate`=7 AND `userid` = #userid), 9000))
when #ptype = 46 then (SELECT 1 + coalesce((SELECT max(`item`) FROM `a1` WHERE `ptype` = 46 AND `plate`=7 AND `userid` = #userid), 10000))
ELSE 0
end) as item
from
a1 WHERE `id1` = #lastid);
UPDATE a1 SET item = #item WHERE id1 = #lastid
as is, #item is returning values of 0 initially, no matter what 'ptype' the new record has, and is incrementing by 1 for subsequent entries.... i need the first record added in each ptype to be 5001 6001, 7001, etc.

Answer 1

First, the answer for which you didn't ask: reverse your idea by creating the rows in their own tables (with an AUTO_INCREMENT as eggyal suggested) and then move the data to this table.
And now the answer:
Your information is a bit mis-matched, which might explain the problem or just be a red herring. For example, you don't describe what 'plate' is, but you use it in your query. You also use #userid, which is not set in your examples.
I created a table that seemed to match your data at the top:
create table a1 (
id1 int primary key auto_increment,
ptype int,
usr int,
item int
);
Then set the variable that you seemed to want:
set #userid = 2;
set #ptype = 43;
and inserted a row:
insert into a1 (ptype, usr) values (#ptype, #userid);
pulled the id back out as you did:
SET #lastid := LAST_INSERT_ID();
Then you can get the max 'item':
select max(item) from a1 WHERE `ptype` = #ptype AND `usr` = #userid;
To handle the initial case, you wanted a default. Since you're separating the ptypes by 1000, you can use that:
SELECT ifnull(max(`item`),(#ptype % 40 + 2)*1000)+1 as next
FROM `a1`
WHERE `ptype` = #ptype
AND `usr` = #userid;
+------+
| next |
+------+
| 5001 |
+------+
Note that this isn't thread safe, so wrap it all in a transaction/trigger.
Hope that helps.

Answer 2

You can use a trigger:
CREATE TRIGGER biA1 BEFORE INSERT ON a1 FOR EACH ROW SET NEW.item = (
  SELECT COALESCE(MAX(item), (NEW.ptype-36)*1000) + 1
FROM a1
WHERE ptype = NEW.ptype AND plate = 7 AND userid = NEW.userid
)