SQL WHERE condition is not equal to?

You can do like this

DELETE FROM table WHERE id NOT IN ( 2 )

OR

DELETE FROM table WHERE id <>  2 

As @Frank Schmitt noted, you might want to be careful about the NULL values too. If you want to delete everything which is not 2(including the NULLs) then add OR id IS NULL to the WHERE clause.

Your question was already answered by the other posters, I'd just like to point out that

 delete from table where id <> 2

(or variants thereof, not id = 2 etc) will not delete rows where id is NULL.

If you also want to delete rows with id = NULL:

delete from table where id <> 2 or id is NULL