Alter a MySQL column to be AUTO_INCREMENT

ALTER TABLE document MODIFY COLUMN document_id INT auto_increment

Roman is right, but note that the auto_increment column must be part of the PRIMARY KEY or a UNIQUE KEY (and in almost 100% of the cases, it should be the only column that makes up the PRIMARY KEY):

ALTER TABLE document MODIFY document_id INT AUTO_INCREMENT PRIMARY KEY

You must specify the type of the column before the auto_increment directive, i.e. ALTER TABLE document MODIFY COLUMN document_id INT AUTO_INCREMENT.

The SQL to do this would be:

ALTER TABLE `document` MODIFY COLUMN `document_id` INT AUTO_INCREMENT;

There are a couple of reasons that your SQL might not work. First, you must re-specify the data type (INT in this case). Also, the column you are trying to alter must be indexed (it does not have to be the primary key, but usually that is what you would want). Furthermore, there can only be one AUTO_INCREMENT column for each table. So, you may wish to run the following SQL (if your column is not indexed):

ALTER TABLE `document` MODIFY `document_id` INT AUTO_INCREMENT PRIMARY KEY;

You can find more information in the MySQL documentation: http://dev.mysql.com/doc/refman/5.1/en/alter-table.html for the modify column syntax and http://dev.mysql.com/doc/refman/5.1/en/create-table.html for more information about specifying columns.

AUTO_INCREMENT is part of the column's datatype, you have to define the complete datatype for the column again:

ALTER TABLE document
ALTER COLUMN document_id int AUTO_INCREMENT

(int taken as an example, you should set it to the type the column had before)

You can apply the atuto_increment constraint to the data column by the following query:

ALTER TABLE customers MODIFY COLUMN customer_id BIGINT NOT NULL AUTO_INCREMENT;

But, if the columns are part of a foreign key constraint you, will most probably receive an error. Therefore, it is advised to turn off foreign_key_checks by using the following query:

SET foreign_key_checks = 0;

Therefore, use the following query instead:

SET foreign_key_checks = 0;
ALTER TABLE customers MODIFY COLUMN customer_id BIGINT NOT NULL AUTO_INCREMENT;
SET foreign_key_checks = 1;

You can use the following query to make document_id to increment automatically

ALTER TABLE document MODIFY COLUMN document_id INT auto_increment

It is preferred to make document_id primary key as well

ALTER TABLE document MODIFY COLUMN document_id INT auto_increment PRIMARY KEY;

Setting column as primary key and auto_increment at the same time:

  mysql> ALTER TABLE persons MODIFY COLUMN personID INT auto_increment PRIMARY KEY;
    Query OK, 10 rows affected (0.77 sec)
    Records: 10  Duplicates: 0  Warnings: 0

    mysql>

Just like this:

alter table document modify column id int(11) auto_increment;

Try the following:

ALTER TABLE table_name MODIFY COLUMN id datatype auto_increment;

Since SQL tag is attached to the question I really think all answers missed one major point.

MODIFY command does not exist in SQL server So you will be getting an error when you run the

ALTER TABLE Satellites MODIFY COLUMN SatelliteID INT auto_increment PRIMARY KEY;

In this case you can either add new column as INT IDENTITY

ALTER TABLE Satellites
   ADD ID INT IDENTITY
       CONSTRAINT PK_YourTable PRIMARY KEY CLUSTERED;

OR

DECLARE @id INT
SET @id = 0 
UPDATE Satellites SET @id = SatelliteID = @id + 1