not working: ALTER TABLE x ADD x IF NOT EXISTS

[foxfirediego]

i'm trying to use a "IF NOT EXISTS" with a install code but i couldn't
could someone correct this for me?
thx

$db->query_write("ALTER TABLE accounts ADD group_id IF NOT EXISTS INT(11) NOT NULL COMMENT 'accounts group' AFTER password");

the error

Database error in vBulletin 3.6.8:

Invalid SQL:
ALTER TABLE accounts ADD group_id IF NOT EXISTS INT(11) NOT NULL COMMENT 'accounts group' AFTER password;

MySQL Error  : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'IF NOT EXISTS INT(11) NOT NULL COMMENT 'accounts group' AFTER password' at line 1
Error Number : 1064