Question

I’m trying to generate a primary key for my table, something like this
(simplified version) – the purpose is to have a daily incremented key:

DELIMITER ^

CREATE TABLE `ADDRESS` (
    ID INTEGER NOT NULL DEFAULT -1,
    NAME VARCHAR(25),
    PRIMARY KEY(`ID`))^

CREATE FUNCTION `GETID`()
RETURNS INTEGER
deterministic
BEGIN
    declare CURR_DATE DATE;
    declare maxid, _year, _month, _day, newid INTEGER;

    set CURR_DATE = CURRENT_DATE;
    set _year = EXTRACT(YEAR FROM CURR_DATE);
    set _mon = EXTRACT(MONTH FROM CURR_DATE);
    set _day = EXTRACT(DAY FROM CURR_DATE);
    set newid = (_year - (_year/100) * 100) * 10000 + _mon * 100 + _day;

    select max(ID) into maxid From `ADDRESS`;
    if (maxid is null) then
        set maxid = 0;
    end if;

    if (MAXID / 1000 != newid) then
        set MAXID = newid * 1000;
    end if;
    set MAXID = MAXID + 1;
    return MAXID;
END^

CREATE TRIGGER `ADDRESS_ID_TRIGGER` BEFORE INSERT ON `ADDRESS`
FOR EACH ROW
BEGIN
    if new.id=-1 then
        set new.id = getid();
    end if ;
END^

COMMIT^

DELIMITER ;

Generally it works fine, but when I test it with multiple inserts at the same time
it obviously fails (e.g. no dirty reads, the select max will fail for the 2nd insert,
thus it will generate the same id as fro the 1st insert).

Answer 1

Workaround:

1. Make primary key AUTO_INCREMENT.
2. Add TIMESTAMP field and use BEFORE INSERT/UPDATE trigget to set CURRENT_TIMESTAMP().

Also you can use ON UPDATE CURRENT_TIMESTAMP option for TIMESTAMP field, value will be updated automatically.

So, ID is ID, and TIMESTAMP field contains date and time.