Referential integrity for invoice and task tables - mysql

Question

I am having trouble figuring out how to design a database for an invoicing system. I want to have an invoice table and an tasks table. I want a record in the invoices table to be able to have a variable amount of tasks associated with it but also keep referential integrity so that a task cannot be added that does not exist on the tasks table.
So far I have come up with a task_id1, task_id2, etc... fields on the invoices table that are foreign keys to the task_id field in the tasks table but it feels limiting in the number of tasks that can be put on one invoice.
Alternatively the invoices table could have a comma separated list of task_id's that would allow for a variable amount of tasks per invoice but I cant figure out how to create the foreign key if the column types are not the same. (Plus this feels a little sloppy).
Im sure there is an easy answer I am not seeing.

Answer 1

So far I have come up with a task_id1, task_id2, etc... fields on the
invoices table that are foreign keys to the task_id field
Alternatively the invoices table could have a comma separated list of
task_id's that would allow for a variable amount of tasks per invoice
NOO!!!
Make a many-to-many tables between invoices and tasks, and make the constraint between this table and both the invoice and task table.
SET #OLD_UNIQUE_CHECKS=##UNIQUE_CHECKS, UNIQUE_CHECKS=0;
SET #OLD_FOREIGN_KEY_CHECKS=##FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0;
SET #OLD_SQL_MODE=##SQL_MODE, SQL_MODE='TRADITIONAL,ALLOW_INVALID_DATES';
CREATE SCHEMA IF NOT EXISTS `mydb` DEFAULT CHARACTER SET latin1 COLLATE latin1_swedish_ci ;
USE `mydb` ;
-- -----------------------------------------------------
-- Table `mydb`.`invoices`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `mydb`.`invoices` (
`id` INT NOT NULL AUTO_INCREMENT ,
`date` DATETIME NULL ,
`amount` DECIMAL(7,2) NULL ,
`whatever` VARCHAR(45) NULL ,
PRIMARY KEY (`id`) )
ENGINE = InnoDB;
-- -----------------------------------------------------
-- Table `mydb`.`tasks`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `mydb`.`tasks` (
`id` INT NOT NULL AUTO_INCREMENT ,
`whatever` VARCHAR(45) NULL ,
PRIMARY KEY (`id`) )
ENGINE = InnoDB;
-- -----------------------------------------------------
-- Table `mydb`.`invoices_has_tasks`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `mydb`.`invoices_has_tasks` (
`invoices_id` INT NOT NULL ,
`tasks_id` INT NOT NULL ,
PRIMARY KEY (`invoices_id`, `tasks_id`) ,
INDEX `fk_invoices_has_tasks_tasks1_idx` (`tasks_id` ASC) ,
INDEX `fk_invoices_has_tasks_invoices_idx` (`invoices_id` ASC) ,
CONSTRAINT `fk_invoices_has_tasks_invoices`
FOREIGN KEY (`invoices_id` )
REFERENCES `mydb`.`invoices` (`id` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_invoices_has_tasks_tasks1`
FOREIGN KEY (`tasks_id` )
REFERENCES `mydb`.`tasks` (`id` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB;
SET SQL_MODE=#OLD_SQL_MODE;
SET FOREIGN_KEY_CHECKS=#OLD_FOREIGN_KEY_CHECKS;
SET UNIQUE_CHECKS=#OLD_UNIQUE_CHECKS;

Answer 2

To create the invoice table
CREATE TABLE invoice (
invoice_id INT NOT NULL,
PRIMARY KEY (invoice_id)
) ENGINE=INNODB;
To create the tasks table use
CREATE TABLE tasks (
task_id INT,
invoice_id INT,
INDEX par_ind (invoice_id),
FOREIGN KEY (invoice_id)
REFERENCES invoice(invoice_id)
ON DELETE CASCADE
) ENGINE=INNODB;
That is all you need