Question: i understand,below `column` will be `signed int` by default. 

```
   id INT(6);

```

Can an `auto increment column` specified below be signed by default? Mysql starts the value from 1 for an auto increment column. 

```
id INT(6) AUTO_INCREMENT PRIMARY KEY

```

Answer: 
Yes, you can create an auto increment primary key with a signed int. Try this:

```
CREATE TABLE mytable( id int(6) AUTO_INCREMENT PRIMARY KEY);

```

Then the following queries are both valid

```
INSERT INTO mytable values();
INSERT INTO mytable values(-10);

```

This will result in the table having a row with -10 and another with 1 as values. But you will run into problems if you try this:

```
 ALTER TABLE mytable AUTO_INCREMENT=-10;

```

yes, you cannot have auto increment values that are negative numbers.
After a lot of searches... I looked for a solution with a TRIGGER called BEFORE INSERT ! I found this : <https://stackoverflow.com/a/43441586/2282880>

Here is my variant :

```
CREATE TRIGGER `invertID` 
BEFORE INSERT ON `<table>`
    FOR EACH ROW SET NEW.id=CONCAT("-", (
        SELECT `auto_increment` 
        FROM INFORMATION_SCHEMA.TABLES    
        WHERE table_name = '<table>')
    )

```

It worked for me fine.

It was the best way I found to sync in both directions two databases with same schema without same ID's in my tables.