Question

I’m trying to create a table that represents an instruction in a recipe:

+---------------------+
| recipeId   (PK, FK) |
| stepNumber (PK)     |
|---------------------|
| instruction         |
+---------------------+

The idea is to have a primary key of (recipeId, stepNumber) where the recipeId comes from the recipe table and the stepNumber auto-increments.

When I tried to create this table, I got the following error:

#1075 - Incorrect table definition; there can be only one auto column and it must be defined as a key 

Is what I’m trying to do correct/possible?

Answer 1

My suggestion is, create the generic id column with auto_increment first, to have a primary key in the table. Then create a unique key for both recipeId and stepNumber together so you won’t have any duplicate combination of these 2 fields.

To be able to add multiple steps for a single recipe you will need to make sure none of recipeId, stepNumber or instruction is set to auto-increment. The only column set to auto_increment remains id.

So the table schema for these 2 tables would look like (ignore the category column)

CREATE TABLE `recipies` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(30) NOT NULL DEFAULT '',
  `category` enum('Salad','Dessert','Meat','Pastry') DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `instructions` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `recipeId` int(11) unsigned NOT NULL,
  `stepNumber` int(11) NOT NULL DEFAULT '1',
  `instruction` text NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `recipeId` (`recipeId`,`stepNumber`),
  CONSTRAINT `instructions_ibfk_1` FOREIGN KEY (`recipeId`) REFERENCES `recipies` (`id`) ON DELETE CASCADE ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Let’s add a record in the recipies table first

INSERT INTO `recipies` (`name`,`category`)
VALUES ('Pumpkin Pie','Pastry');

Then let’s add a row

INSERT INTO `instructions` (`recipeId`,`instruction`,`stepNumber`)
SELECT
    1,
    'You will need plenty of pumpkins!',
    IFNULL(MAX(`stepNumber`),0)+1
FROM `instructions`
WHERE `recipeId`=1

• 1 after SELECT and 1 in the WHERE condition both refer to the row with id=1 in the recipies table
• IFNULL(MAX(stepNumber),0)+1 will select the highest step number for that recipe (if it doesn’t exist it will select “0”) +1

Here’s a SQL fiddle if you want to see it working.

[EDIT]
I have never needed using a combo for the primary key but apparently following works on InnoDB provided you don’t already have a primary key in the table.

ALTER TABLE `instructions`
ADD PRIMARY KEY(`recipeId`,`stepNumber`)