Mysql – Create MySQL CHILD and PARENT records at the same time and set CHILD.PARENT_ID by using MySQL Triggers

mariadbMySQLtrigger

I want to create relationships between parent and child when booth doesn't exist and when new child is created. This somehow can be done with Triggers but i don't know how to work with SQL.

After some googling i came up with Trigger concept bellow.

  1. Create child
  2. Create parent
  3. Get last parent id
  4. Update CURRENT child record with parent_id

Could somebody help me to achieve this?

DELIMITER $$
CREATE
    TRIGGER `database_name`.`trigger_name` AFTER INSERT
    ON `database_name`.`child_table`
    FOR EACH ROW BEGIN

    /*Step 1 - Create Parent record.*/
    INSERT INTO  `database_name`.`parent_table`(id) VALUES (``);

    /*Step 2 - Get last inserted ID of just created Parent record 
      and assign to the variable*/
    DECLARE last_id int;

    SELECT LAST_INSERT_ID INTO last_id;
    FROM information_schema.tables
    WHERE table_name = `parent_table`
    AND table_schema = `database_name`;

    /*Step 3 - Update current record child_table.parent_id with last_id variable*/
    UPDATE NEW.parent_id = last_id;
END$$
DELIMITER ;

Or something like this, but it has errors.

DELIMITER $$

DROP TRIGGER IF EXISTS create_new_child; $$

CREATE 
    TRIGGER create_new_child BEFORE INSERT
    ON child
    FOR EACH ROW BEGIN
    DECLARE last_id INT;
        SELECT AUTO_INCREMENT INTO last_id
        FROM information_schema.TABLES
        WHERE TABLE_NAME = parent
        AND TABLE_SCHEMA = DATABASE();
    INSERT INTO parent (title) VALUES (`TEST`);
    SET NEW.parent_id = last_id;
    END$$
DELIMITER ;

Unknown column 'parent' in 'where clause'

Best Answer

I think you have to perform this trigger before insert child row and there is a lot of errors in your code.

  1. You must declare variables in the beginning of the script.
  2. You give values to variables with SET
  3. Only LAST_INSERT_ID() function is necessary and if you perform the trigger before update, you only have to set NEW value, and that's it.

DELIMITER $$
DROP TRIGGER `database_name `.`trigger_name` $$
CREATE
    TRIGGER `database_name `.`trigger_name` BEFORE INSERT
    ON `database_name `.`child_table`
    FOR EACH ROW BEGIN
    DECLARE last_id int;
    INSERT INTO  `database_name`.`parent_table`(id) VALUES (0);

    SET NEW.parent_id = LAST_INSERT_ID();
END $$
DELIMITER;
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