MySql IF statement in query

I just finished picking up my jaw off the floor after learning that you can't use an IF statement in a query in MySql. How can anyone get anything done without an IF statement?!

What I'm trying to do is write a DML script that adds a constraint to a table, if it doesn't exist. Something like this:

if (select count(*) from information_schema.table_constraints
    where constraint_name='开发者_开发技巧fk_user_user_status') = 0
then
    alter table `user`
        add constraint fk_user_user_status foreign key (status_id)
            references user_status(id);
end if;

How would one do this in MySql?

Thanks in advance!

You can only run IF statements in stored procedures and functions.

http://dev.mysql.com/doc/refman/5.0/en/if-statement.html

If you are executing standard scripts, then IF statements only work with SELECT queries.

http://dev.mysql.com/doc/refman/5.0/en/control-flow-functions.html#function_if

Procedural batch, but within the below, a proc is created and dropped after the operation completes.

delimiter $$

drop procedure if exists tmp_add_fk $$

create procedure tmp_add_fk()
begin
if (select count(*) from information_schema.table_constraints
    where constraint_name='fk_user_user_status') = 0
then
    alter table `user`
        add constraint fk_user_user_status foreign key (status_id)
            references user_status(id);
end if;
end$$

delimiter ;

call tmp_add_fk;

drop procedure tmp_add_fk;