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Description

This error occurs when creating or altering a MySQL table with an `AUTO_INCREMENT` column that violates fundamental definition rules. Specifically, it indicates that you've either defined more than one `AUTO_INCREMENT` column or the single `AUTO_INCREMENT` column is not properly defined as part of a key.

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Error Message

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

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Known Causes

2 known causes

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Multiple AUTO_INCREMENT Columns

The table definition attempts to assign the `AUTO_INCREMENT` attribute to more than one column. MySQL permits only a single `AUTO_INCREMENT` column per table.

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AUTO_INCREMENT Lacks Key Definition

The column designated as `AUTO_INCREMENT` is not defined as part of a `PRIMARY KEY` or `UNIQUE KEY` constraint. MySQL requires `AUTO_INCREMENT` columns to be indexed.

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Solutions

4 solutions available

1. Fix AUTO_INCREMENT Column Position easy

AUTO_INCREMENT must be first column in a key

1

AUTO_INCREMENT requires PRIMARY KEY or first in key

-- Wrong:
CREATE TABLE items (
  id INT AUTO_INCREMENT,
  name VARCHAR(100)
);

-- Right:
CREATE TABLE items (
  id INT AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(100)
);

2

Or add as first column in composite key

CREATE TABLE items (
  id INT AUTO_INCREMENT,
  category VARCHAR(50),
  name VARCHAR(100),
  PRIMARY KEY (id, category)
);

2. Fix Conflicting Table Options medium

Remove incompatible table settings

1

Check for conflicting ROW_FORMAT and features

-- Some features require specific row formats:
-- COMPRESSED requires ROW_FORMAT=COMPRESSED
-- Large VARCHAR/TEXT columns may need DYNAMIC

CREATE TABLE posts (
  id INT AUTO_INCREMENT PRIMARY KEY,
  content LONGTEXT
) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;

3. Fix TIMESTAMP/DATETIME Issues medium

Multiple auto-update timestamps may conflict

1

Only one TIMESTAMP can have automatic properties (MySQL < 5.6)

-- Wrong in MySQL < 5.6:
CREATE TABLE posts (
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);

-- Right for older MySQL:
CREATE TABLE posts (
  created_at DATETIME DEFAULT NULL,
  updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);

4. Check for Circular References medium

Foreign key definitions may conflict

1

Create tables in correct order

-- Create parent table first
CREATE TABLE categories (
  id INT AUTO_INCREMENT PRIMARY KEY
);

-- Then child table with FK
CREATE TABLE products (
  id INT AUTO_INCREMENT PRIMARY KEY,
  category_id INT,
  FOREIGN KEY (category_id) REFERENCES categories(id)
);

2

For circular references, add FK after creation

CREATE TABLE employees (id INT PRIMARY KEY, manager_id INT);
ALTER TABLE employees ADD FOREIGN KEY (manager_id) REFERENCES employees(id);

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Related Errors

5 related errors

MySQL Error 1075: Invalid AUTO_INCREMENT Setup