MySQL Insert/Update Trigger with AUTO_INCREMENT

Question

So, I've got a table roughly as follows:

CREATE TABLE CUSTOMER (
    CUSTID              INT NOT NULL AUTO_INCREMENT,
    NAME                CHAR (45),
    CONSTRAINT CUSTOMER_PRIMARY_KEY PRIMARY KEY (CUSTID))
AUTO_INCREMENT = 100;

I'm auto incrementing the CUSTID so that it's possible to simply insert a name and have it created with the next available CUSTID . However, I also want to ensure that it isn't possible to set the CUSTID value to zero, either on creation of the row or on update so I've constructed the following trigger:

DELIMITER $$
CREATE TRIGGER `custid_before_insert` BEFORE INSERT ON `CUSTOMER`
FOR EACH ROW
BEGIN
    IF (NEW.CUSTID) <= 0 THEN
        SIGNAL SQLSTATE '12345'
            SET MESSAGE_TEXT = 'Check constraint on CUSTOMER.CUSTID failed';
    END IF;
END$$

CREATE TRIGGER `custid_before_update` BEFORE UPDATE ON `CUSTOMER`
FOR EACH ROW
BEGIN
    IF (NEW.CUSTID) <= 0 THEN
        SIGNAL SQLSTATE '12345'
            SET MESSAGE_TEXT = 'Check constraint on CUSTOMER.CUSTID failed';
    END IF;
END$$
DELIMITER ;

Unfortunately in my blissful ignorance of how AUTO_INCREMENT worked, I've come to the conclusion that this is the wrong way to go about this. Trying to insert a customer with no CUSTID value is tripping the trigger causing the insert to fail which I presume is due to the value being a zero before insertion when AUTO_INCREMENT assigns it a value.

Would the best way to do this really be to change the trigger to occur after the insert and delete the row or is there a better way to do this to just throw an error?

Answer 1

The insert trigger is not needed.

From Auto_Increment

No value was specified for the AUTO_INCREMENT column, so MySQL assigned sequence numbers automatically. You can also explicitly assign 0 to the column to generate sequence numbers.

EG

create table t(id int auto_increment, primary key(id));
insert into t(id) values (0);
select id from t;
# 1

Update:

To allow the insert to complete when CUSTID is not specified,

INSERT INTO customer(name) VALUES('Chuck');

check for null in the trigger:

IF NEW.CUSTID IS NOT NULL AND NEW.CUSTID <= 0 THEN

Answer 2

Inserting '0' into an auto-increment column causes it to increment the same as inserting NULL, so you really neither need nor want the INSERT trigger. Try it with just the UPDATE trigger.