How to create custom column with auto-increment characters

问题

I want to show one custom column as alias but need to increment by using auto character.

id      subid   dollar  packetname
168     355     5813    ND-1
169     355     359     ND-1
170     356     559     ND-2
171     362     4536    ND-10
172     362     484     ND-10
134     329     4698    ND-12
135     329     435     ND-12
125     330     6293    ND-13
126     330     4293    ND-13
127     330     693     ND-13

I need a output with another updated packet. column with autoincrement character

id      subid   dollar  packetname      updated packet
168     355     5813    ND-1            ND-1
169     355     359     ND-1            ND-1A
170     356     559     ND-2            ND-2
171     362     4536    ND-10           ND-10
172     362     484     ND-10           ND-10A
134     329     4698    ND-12           ND-12
135     329     435     ND-12           ND-12A
125     330     6293    ND-13           ND-13
126     330     4293    ND-13           ND-13A
127     330     693     ND-13           ND-13B

回答1:

You can use such query to make additional field

SELECT concat(packetname, 
              elt(if(@t=packetname, @n:=@n+1, @n:=1),
                  '','A','B','C','D','E','F','G')) `updated packet`, 
       id, subid, dollar, @t:=packetname packetname
    FROM t
      cross join
         (SELECT @n:=1, @t:="") n
  order by packetname

demo on sqlfiddle

回答2:

I think your best bet is to SELECT out the current packet table, modifying it along the way, and INSERT it into a new table. Once you have completed this operation, you can drop the original table, and then rename the new one to the old one.

INSERT INTO newpacket (id, subid, dollar, packetname, `updated packet`)
SELECT p1.id, p1.subid, p1.dollar, p1.packetname, p2.`updated packet`
FROM packet p1
INNER JOIN
(
    SELECT p.id, p.subid,
        CASE WHEN (SELECT p.id - MIN(t.id) FROM packet t WHERE t.subid = p.subid) > 0
        THEN CONCAT(packetname,
                    CHAR(((SELECT p.id - MIN(t.id) FROM packet t WHERE t.subid = p.subid) + 64) USING utf8))
        ELSE packetname END AS `updated packet`
    FROM packet p
) p2
ON p1.subid = p2.subid AND p1.id = p2.id

回答3:

You can create a trigger.

create trigger my_trigger before insert on mytable for each row 
begin
DECLARE samecount INT;
  set samecount = ( select count(*) from mytable where packetname = new.packetname );
  if samecount = 0 then
    set new.updated_packet = new.packetname;
  else
    set new.updated_packet = concat(new.packetname,conv(samecount+9,10,36));
  end if;
end;

Before a new row is inserted, it counts how many rows with same packetname exist. When there is one or more, the count+9 is converted to base 36 - it's almost the same as HEX except all the way up to Z. So, if count is 1, it becomes 1+9=10=A. The resulting value is concatenated with packetname. If same rows exceed 37, it will not fail, but will append 10 for 38 instead.

Keep in mind that this isn't exactly auto increment and it could be subject to race conditions, when two users insert the same packetname in exactly the same time, the count query can return the same value for both.

EDIT: Note that this solution for when you'll need to insert new rows into that table afterwards and want them to have updated_packet filled automatically. If you want to also update existing rows, one way is to create a new table with same structure, create that trigger on a new table and then do

insert into newtable(id, subid, dollar, packetname)
select id, subid, dollar, packetname from oldtable

来源：https://stackoverflow.com/questions/34329910/how-to-create-custom-column-with-auto-increment-characters

标签

mysql

auto-increment