如何更新 Mysql 触发器中的 LAST_INSERT_ID

Question

In my MySQL Database, I have a table with a composite primary key where the ID is not in auto_increment mode. Something like this:

CREATE TABLE table_a (
    fk_table_b INT UNSIGNED NOT NULL,
    id INT UNSIGNED,
    label VARCHAR(80) NOT NULL,
    PRIMARY KEY (fk_table_b, id),
    FOREIGN KEY fk_table_b
        REFERENCES table_b(id)
);

To increment the ID in function of the foreign key, I made a trigger like this:

DELIMITER $$
CREATE TRIGGER table_a_auto_increment
BEFORE INSERT ON table_a
FOR EACH ROW BEGIN
    SET NEW.id = (
        SELECT IFNULL(MAX(id), 0) + 1
        FROM table_a
        WHERE table_a.fk_table_b  = NEW.fk_table_b
    ); 
END $$
DELIMITER ;

But when I do SELECT LAST_INSERT_ID() I am getting 0 as the new id... Normally you could override the LAST_INSERT_ID() by giving it a number like this:

INSERT table_a ( fk_table_b, id)
VALUES (1, LAST_INSERT_ID(5));
SELECT LAST_INSERT_ID(); -- -> it gives me 5

So I have tried to combine both to do this trigger:

DELIMITER $$
CREATE TRIGGER table_a_auto_increment
BEFORE INSERT ON table_a
FOR EACH ROW BEGIN
    SET NEW.id = (
        SELECT LAST_INSERT_ID(IFNULL(MAX(id), 0) + 1)
        FROM table_a
        WHERE table_a.fk_table_b  = NEW.fk_table_b
    ); 
END $$
DELIMITER ;

But it's still giving me 0 when I insert something in the base... Do you know if there is a way to make it work?

Thanks a lot.

-- EDIT 2020-08-14

Finally it seems impossible to override the LAST_INSERT_ID function inside the TRIGGER, so I changed my solution by removing the trigger and doing it inside my insert function like this:

INSERT table_a ( fk_table_b, id, label)
VALUES (1, LAST_INSERT_ID((
    SELECT IFNULL(MAX(old_one.id), 0) + 1
    FROM table_a AS old_one
    WHERE old_one.fk_table_b  = table_a.fk_table_b
)), "something");

And then, this is giving me the good result I can use in my backend:)

Answer 1

You may use additional service table:

CREATE TABLE service_table (id BIGINT AUTO_INCREMENT PRIMARY KEY);

and

DELIMITER $$
CREATE TRIGGER table_a_auto_increment
BEFORE INSERT ON table_a
FOR EACH ROW 
BEGIN
    SET NEW.id = (
        SELECT IFNULL(MAX(id), 0) + 1
        FROM table_a
        WHERE table_a.fk_table_b  = NEW.fk_table_b
    ); 
    DELETE FROM service_table WHERE id IS NOT NULL;
    INSERT INTO service_table VALUES (NEW.id - 1);
    INSERT INTO service_table VALUES (NULL);
    SET NEW.id = LAST_INSERT_ID() - 1;
END $$
DELIMITER ;

fiddle (foreign key removed).

---

1. Maybe the code may be simplified a little - do it yourself.
2. Service table may be defined as Engine = MEMORY (if available).
3. The code is not safe for concurrent inserts.