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.